The "resolve" function in Fexl now takes a boolean argument "strict".

The resolve function always resolves any symbols for which it can find
definitions, and leaves any undefined symbols remaining in the final form.
If strict is true, it reports undefined symbols to stderr and exits with code
1 if it saw any.  If strict is false, it does none of that, and simply returns
the final form with any undefined symbols still in it.  This lets you do
partial resolution of forms, resolving some symbols but not others.

Add built-in constants "T", "no", and "@".

Renamed these variables:
	source_fh   => curr_fh
	source_name => curr_name
	source_line => curr_line

Don't need to use special tokens case_no and case_yes -- just use whatever
combinators are appropriate (e.g. T/F or yes/no).