Add compile time Benchmarks

[schwern]

Makes sure this compiles significantly faster than Moo, Moose and Mouse.

Put it in xt/ or whatever the convention for author tests is in Module::Build.

[tobyink]

Some benchmarks can be seen here:
https://metacpan.org/dist/Mite/view/lib/Mite/Manual/Benchmarking.pod

Mite is actually a little slower to load than Moo and Mouse, probably because it needs to load two .pm files per class instead of one. Still significantly faster than Moose though.