Make the entire processor pipeline work with streams

[maanlamp]

For testing, we're processing small files. When the language will be used in bigger projects, it will not scale if it has to load and save everything into memory. Streams were introduced exactly for this use case, so we should use them.

[maanlamp]

Lexical analysis of streams turns out to be more trouble than it's worth (sometimes errors at valid code, sometimes doesn't error at invalid code), so we're postponing this for when it is actually needed. Maybe reopen this issue then. To allow for the change at any time, we are going to use a new generator function pattern to export tokens from the lexer.