Improve consistency and usability of void type and stmt/expr distinction

[kodek16]

(a.k.a. the void-sanity tracking issue)

The void type and statement vs. expression syntax right now is quite easy to trip on. Missing semicolon errors point to the next line which is most often unrelated to the issue. Both in the compiler internals and various documentation, void and void-expressions stick out as a sore thumb.

There are a number of things that need to be done to address it:

• Make semicolons optional.
• Automatically promote expressions to statements when and only when implied by context.
• Consider removing void type from program IR. This would prevent a lot of confusion and potential void-related errors, but might require considerable code duplication (IfExpr/IfStmt, BlockExpr/BlockStmt, function calls, maybe something else?).