Tighten parser

[sophiajt]

The parser is currently pretty loosey-goosey about strictness to any particular language design. For example, you can elide semicolons between adjacent statements. This makes for some confusing issues when a script refuses to run and it's because you missed something that's hard to see.

Before 1.0, it'd be nice to have a pretty straightforward set of grammar rules that are enforced by the parser.