Shen-C 0.2.3

• Updated Shen version to 22.4
• Allow absvector to create an empty vector
• Added a shell script shen-c to allow running Shen-C without setting the environment variable SHEN_C_HOME
• Added a new Makefile target to create a release build

Shen-C 0.2.1

• Removed unnecessary extensions
• Use c package for port-specific extensions

Shen-C 0.2.0

Bump version to 0.2.0