Fix a number of issues with the parser

[dcbaker]

The biggest change here is that we actually check the return value of the lexer and parser, which means that a number of things that were invalid are now actual errors.

I've also fixed a number of issues that I discovered by running most of the meson.build files from the main meson repo through the standalone parser:

• escape sequences in strings (but not triple strings yet)
• foreach loops with dictoinaries
• incorrect counting of lines
• using backslash to escape a newline