Replace mustache_test with generated spec

This replaces the `mustache_test` file with a much more comprehensive test suite generated from [the official Mustache spec] (https://github.com/mustache/spec). I wrote [a little generator] (https://github.com/adammck/mustache_spec_generator) to do this rather than parsing the spec and generating the tests at runtime, because I think the output is surprisingly idiomatic and comprehensible.

This improved test suite causes **about 50 failures, a runtime panic and an infinite loop**, so you might want to look into those before merging. (Thankfully most of them are related to whitespace and error messages, so should be simple to fix.) I've already started working on those [in my fork] (https://github.com/adammck/mustache/tree/fix_tests), and can update this PR when they're all fixed.