Add export keyword handling.

[jonmeow]

This provides export import logic in lex, parse, and check; export name logic is only in lex and parse, not check.

I think with export name I'm going to need to modify import_ref and some consolidation logic, whereas export import seems feasible to keep as primarily import logic. Given the implementations were looking like they'd diverge more substantially, I thought it'd be helpful to cut the PR here.