- a single hyphen is a string, not a number.
- a string with more than one period is a string, not a number
- check for string overflow
Add some unit tests
Since now we can distinguish between def and use according to
the variable's location, there is no need to keep two variable
prefixes.
Also reformat tests to use the value generating instruction
format ("<result-id> = <opcode> <operand>..").
Fix the bug that TextAdvance() forgot to skip whitespace at the
beginning of the next line after a comment line.
Fix the bug that TextAdvanceLine() increase line number after going
over a character.
