Simplify comparison function specification
Use enums rather than strings

I sympathise with the ergonomic argument, but this is simpler, less error prone, and more efficient
Rename Constructor->Builder
Move embed_to onto Mesh

This makes more sense, because it is now applied to self, rather than an argument.
Rename FrontType and variant names for clarity
Follow Rust guidelines for naming and add documentation
Use a type alias for the byte size
Fix travis build
Merge pull request #106 from bjz/api-cleanups

Api cleanups