Add bounds checking for constant expression contexts

[kevinhartman]

When a reference to an expression appears in the assembly, it can be resolved in the second pass if the expression does not contain external references. The context in which the reference appears will usually have a size requirement.

For example, in the case of a dc.w <expr> directive, the value of the expression must fit in a word. We should add limits checking to these expression operands so that if they're resolved in the second pass, the appropriate error is thrown if the result does not fit.