Speed up listing files in large directory, README.adoc typo fixed

[dikdom]

When calling the ::openNextFile(), it iterates though the current directory one by one and when it find a usable return value (file or directory) tried to create a new SdFile instance by name which scans the whole directory again for that file. Though the entry is already found by the openNextFile().
Index is calculated based on current file position and SdFile is created based on index.

[github-actions]

Memory usage change @ a23ede4

Board	flash	%	RAM for global variables	%
arduino:avr:leonardo	🔺 0 - +182	0.0 - +0.63	0 - 0	0.0 - 0.0
arduino:avr:mega	🔺 0 - +182	0.0 - +0.07	0 - 0	0.0 - 0.0
arduino:avr:nano	🔺 0 - +182	0.0 - +0.59	0 - 0	0.0 - 0.0
arduino:mbed_nano:nano33ble	🔺 0 - +64	0.0 - +0.01	0 - 0	0.0 - 0.0
arduino:mbed_nano:nanorp2040connect	🔺 0 - +88	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:mbed_portenta:envie_m4	🔺 0 - +64	0.0 - +0.01	0 - 0	0.0 - 0.0
arduino:mbed_portenta:envie_m7	🔺 0 - +64	0.0 - +0.01	0 - 0	0.0 - 0.0
arduino:megaavr:nona4809	🔺 0 - +182	0.0 - +0.37	0 - 0	0.0 - 0.0
arduino:sam:arduino_due_x_dbg	🔺 0 - +80	0.0 - +0.02	N/A	N/A
arduino:samd:mkrzero	🔺 0 - +92	0.0 - +0.04	0 - 0	0.0 - 0.0

Click for full report table

Board	examples/CardInfo flash	%	examples/CardInfo RAM for global variables	%	examples/Datalogger flash	%	examples/Datalogger RAM for global variables	%	examples/DumpFile flash	%	examples/DumpFile RAM for global variables	%	examples/Files flash	%	examples/Files RAM for global variables	%	examples/NonBlockingWrite flash	%	examples/NonBlockingWrite RAM for global variables	%	examples/ReadWrite flash	%	examples/ReadWrite RAM for global variables	%	examples/listfiles flash	%	examples/listfiles RAM for global variables	%
arduino:avr:leonardo	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	182	0.63	0	0.0
arduino:avr:mega	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	182	0.07	0	0.0
arduino:avr:nano	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	182	0.59	0	0.0
arduino:mbed_nano:nano33ble	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	64	0.01	0	0.0
arduino:mbed_nano:nanorp2040connect	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	88	0.0	0	0.0
arduino:mbed_portenta:envie_m4	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	64	0.01	0	0.0
arduino:mbed_portenta:envie_m7	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	64	0.01	0	0.0
arduino:megaavr:nona4809	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	182	0.37	0	0.0
arduino:sam:arduino_due_x_dbg	0	0.0	N/A	N/A	0	0.0	N/A	N/A	0	0.0	N/A	N/A	0	0.0	N/A	N/A	0	0.0	N/A	N/A	0	0.0	N/A	N/A	80	0.02	N/A	N/A
arduino:samd:mkrzero	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	92	0.04	0	0.0

Click for full report CSV

Board,examples/CardInfo<br>flash,%,examples/CardInfo<br>RAM for global variables,%,examples/Datalogger<br>flash,%,examples/Datalogger<br>RAM for global variables,%,examples/DumpFile<br>flash,%,examples/DumpFile<br>RAM for global variables,%,examples/Files<br>flash,%,examples/Files<br>RAM for global variables,%,examples/NonBlockingWrite<br>flash,%,examples/NonBlockingWrite<br>RAM for global variables,%,examples/ReadWrite<br>flash,%,examples/ReadWrite<br>RAM for global variables,%,examples/listfiles<br>flash,%,examples/listfiles<br>RAM for global variables,%
arduino:avr:leonardo,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,182,0.63,0,0.0
arduino:avr:mega,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,182,0.07,0,0.0
arduino:avr:nano,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,182,0.59,0,0.0
arduino:mbed_nano:nano33ble,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,64,0.01,0,0.0
arduino:mbed_nano:nanorp2040connect,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,88,0.0,0,0.0
arduino:mbed_portenta:envie_m4,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,64,0.01,0,0.0
arduino:mbed_portenta:envie_m7,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,64,0.01,0,0.0
arduino:megaavr:nona4809,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,182,0.37,0,0.0
arduino:sam:arduino_due_x_dbg,0,0.0,N/A,N/A,0,0.0,N/A,N/A,0,0.0,N/A,N/A,0,0.0,N/A,N/A,0,0.0,N/A,N/A,0,0.0,N/A,N/A,80,0.02,N/A,N/A
arduino:samd:mkrzero,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,92,0.04,0,0.0