Merge escape and character AST node into one value node. 

Also, renamed the `name` property to `kind`.

This fixes #33.
Here’s what I meant:

`\uXXXX` has `kind: 'unicode'`. Let’s change that into `kind: 'unicodeEscape'`.
`\u{XXXX}` has `kind: 'codePoint'`. Let’s change that into `kind: 'unicodeCodePointEscape'`.
`\xAA` has `kind: 'hex'`. Let’s change that into `kind: 'hexadecimalEscape'`.