Change API directory structure

This modifies the API directory structure to put chisel3 API directly
under chisel-lang/api/ (instead of under chisel-lang/api/chisel3/).
This also removes all "v" specifies in API paths, e.g., "v3.0.0" is
now "3.0.0".

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>