eolian: more intuitive generation options and mrge impls into .c

eolian: more intuitive generation options and mrge impls into .c

Previously the output base name used to include extension, now
you don't need to specify an extension and it's determined from
the input file name instead.

Also, implementation boilerplate used to merge with .eo.c before,
which made no sense. Now it's merged with .c instead when it
exists or makes a new .c file when it doesn't.