Format preprocessed token stream in multiple passes

Currently, the formatter doesn't handle many scenarios involving preprocessor
`ifdef`s/`endif`s interleaved with `begin`s, module headers, etc. (#228, #241, #267)
This patch attempts to solve this problem by performing multiple passes of the
formatting on preprocessed variants of the source. Each of these variants has a
different set of preprocessor branches enabled. Together, they should cover the
entire source (though that doesn't work in all cases yet). After several
formatting passes for different variants of the AST, a correct and properly
formatted file is produced.

This is still work in progress, so not everything works, and the code isn't very
clean. I'd love to get some early feedback on this.

Signed-off-by: Krzysztof Bieganski <[email protected]>