spec: bytes data type

This change adds initial specification of the bytes data type
following length discussion in #112.
It also explains the implementation-dependent encoding of
text strings, and the \u \U \X escapes.

More will follow, but let's get the easy parts out of the way first.

Updates #112

Change-Id: I8cfbb4910c2f85a1076f9b8bdf1081c89dd5948a