operations to implement in expreval

[metagn]

Hopefully expreval will not need to be a full fledged VM but there are still operations we probably need to support for basic const evaluation.

Implemented as magics:

• arithmetic, comparison
• numeric type conversions, casts
• sizeof etc
• set ops

Implemented as procs in system but will probably be specialized in expreval:

• string ops, maybe just basic ones like &, len, comparisons, as strings are represented by string literal tokens in expreval
• seq ops and openarrays are probably not used too commonly
• maybe set ops can be implemented as procs and overriden in expreval as well

These could be implemented via pragmas like {.vmop: "string.len".} or by hard coding the symbol names in the compiler which might get annoying. But the pragmas would probably build a table from symbols to their respective operations anyway, just at compiler runtime.

Pure function calls in consts/array length exprs are also common but most likely not necessary for the standard library. Templates should work though.