Omit grammar table entries which have no verb, with a warning

[erkyrath]

See issue https://github.com/DavidKinder/Inform6/issues/205 .

To support this, we add two fields to the verbt structure: a flag for whether the verb is used, and the source location where the verb was originally declared.

The flag is set late in compilation, in the locate_dead_grammar_lines() function. We generate warnings at this time. The flag is then checked during game file generation in tables.c.

Miscellaneous changes:

• Move the declaration of struct brief_location_s earlier in header.h.
• We didn't have a warning_at(char*, location) function. Now we do.
• The code that read English_verb_list in find_verb_by_number() was incorrectly used signed arithmetic. The --trace VERBS output was therefore incorrect with more than 128 verbs. (Only affected trace output, not compiled code.)
• We use 16-bit fields for verb number (both Z-code and Glulx, and also in the compiler itself). So more than 65535 verbs will cause problems. Added checks for this.
• We now support --trace MAP=3, which shows the byte size of each memory segment in addition to the start address, end address, and percentage size.

[erkyrath]

Testing: there's a trim-grammar branch in https://github.com/erkyrath/Inform6-Testing/tree/trim-grammar .