Assembler: a string that spells an instruction is no longer assembled as one

A token is classified after its quotes have been stripped, so a string
literal reading "ADD" looked exactly like the ADD instruction and was
assembled as one. It failed with "attempting to assemble outside the
Program Segment", a message about a mistake nobody had made.

The literal and label checks were already guarded against strings and the
instruction check was not. Mnemonics match without regard to case, so
"or" and "and" were caught by this too, and those are ordinary enough
words to want in a message.

Third of its family, after a string beginning with '0' being read as a
malformed number and a string in the Program Segment being silently
discarded. All three have the same root.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Tests/expected/stringMnemonic.out

@@ -0,0 +1,3 @@
+ADD OR and NOP
+Execution halted.
+[exit 0]