Guard integer values

[bgins]

We need to guard integer values based on their size and signedness (see section 6.3.1).

Signedness is determined by the operations performed on an integer (section 2.3.1), though integers can have an explicit + or - to indicate they are signed. At the moment, the lexer treats integers as uN tokens or sN tokens if the have a + or - prefix.

Checking the instruction applied to an integer will likely give us the information we need to determine size and signedness, but this needs further investigation.