Link-Time Address, Bank, Alignment Calculations

[Hawkbat]

See gbdev/rgbds#244

Extend the object file format to allow link-time expressions for section address, bank, and alignment, e.g. section "Some Section", ROMX[$FF00 + LOW(Some.Label)], BANK[BANK("Other Section")], ALIGN[2 + 3]. Alignment isn't really necessary but it feels weird to exclude it.

Additionally, the most common use-case BANK[BANK("Other Section")] should be able to be written simply as BANK["Other Section"]; in other words, if the expression evaluates to a string rather than a number.