Code cleanup

[erkyrath]

Changes from @heasm66, somewhat tidied up. These allow the code to pass extra-super-careful compile-time checks.

• The error message "Opcode unavailable in this Z-machine version" safety-checks the opcode value before printing. (Utility function opcode_name_z().)
• isdigit(), isalnum(), etc macros wants argument cast to uchar.
• In Infix code in asm.c, use INITAO() and related macros to initialize assembly_operand structs.
• In --trace asm output, when a Z-string has abbreviation marks, print "<ABBR>" rather than ctrl-A characters.
• Replace hardcoded constant 35 with MAX_TRACE_STRING_LEN.
• Added /* Fall through */ comments to silence compiler warnings.
• Added line breaks between statements to silence compiler warnings.
• Changed some local variables to uint32 and added casts where necessary.
• Added return; after a couple of compiler_error() calls.
• Changed ASSERT_ZCODE() and ASSERT_GLULX() macros to complete no-ops rather than 0 expressions.