eolian gen: generate a separate file with legacy implementations
If legacy implementations are generated, eolian_gen now creates
a separate file for them and includes it. The file name convention
for that is <.c file name without ext>.legacy.<.c file name ext>
or <.c file name>.legacy.c if it has no ext, so for example
foo.eo.c becomes foo.eo.legacy.c and foo becomes foo.legacy.c.
The file is generated assuming there are any legacy APIs contained
inside the .eo file.