Merge next branch into master

[brendanzab]

I've just published the next branch and opened #197 which marks the beginning of my work to rebuild the current type checker on a better, more efficient, easier to understand foundation - ie. using normalization by evaluation. I've learned a huge amount since I implemented Pikelet originally, and this is the result of that!

Closes #14, #62, #109, #181.

Todo

In order to get to a stage where we are on-par with our current implementation, I'll still need to do a number of things:

• implement comments (using a custom lexer) (Next branch with logos lexer #199)
• implement dependent record types
• implement dependent function types (Implement dependent function types #200)
• implement let expressions
• type annotations on record terms
• pretty print terms with the correct precedence (af71434)
• use a pretty error reporting library like codespan or annotate-snippets
• implement imports
• clean up the documentation page
• fix function eliminations

...I might come up with some other things though later on!