Function esSafeId

Converts arbitrary string to string safe to be used as ECMAScript identifier.

1. Converts empty string to _.
2. Converts [reserved word] to the form __${id}__.
3. Encodes subsequent Unicode code points unsafe to be used in [identifiers] as a series of _x${hex}x${hex}..._ symbols.
4. Does not modify ECMAScript-safe identifiers.