Support Unicode encodings

[mholt]

The encoding currently only supports ASCII (oops - forgot to upgrade that) since each character only gets 1 byte. This should be expanded to more bytes when the user wants to encode trees with Unicode characters.

However, since this greatly increases the file size and may not be necessary, so this option should be configurable.

[sqs]

Just ran into this issue (it was causing a stack overflow--a "stack growth" panic). I'm fine with just using ASCII, for my use cases, but would you accept a PR that returned an error or panicked in Insert if non-ASCII is added?

[mholt]

Hey Quinn, I'm sure we'd love a PR for that. It's a decent stop-gap. @mdwhatcott or @joliver might be able to merge it in for ya.