A dropped datum is replaced by whatever follows it, but what follows
may be the end of the file or the paren closing the list we are
in. Hand the token back to the caller instead: read-list closes its
list with it and the toplevel loop stops.
A `;' comment between a guard and the form it guards was also taken for
the guarded datum, so the form stayed unconditional and the guard did
nothing. Skip comments when reading the guard.
comment-form? was defined three times over; it moves to utils.
For source-level debug. Add --line-directives option, defaulted to
statement. Disabled when used with -C. Explicit non-none value
overrides disabling when -C is present
Also rename macros to sex-macros, module-system to sex-modules for
clarity, uniformity, and to avoid name clashes with Chicken's
units/modules named "macros" and "modules"
Introducing Sex SEMantic ENgine: the semen.
Also split reader to other file (it can be replaced in the future).
Macro expansion inside Sex code doesn't work yet, and it must be done
in semen, not during fmt-c generation as before.
