Expose util functions on scripts & buffers.

Correction: buffer's codepoints are all Chars.

Reimplement parts of Harfbuzz where that's easier than writing a binding.

Removed a note where that turned out not to be the case.

Integrate guessing chars, document properties, expose default property values.

Implement conversion from functional Buffers to imperative Buffers.

Draft pure-functional Harfbuzz API.