Utf8 mask sum test changed to assert.
UTF-16: Decoding unpaired lead surrogate now correctly fails.
UTF-16: Changed mask sum test to assert.
UTF-16: Added notes about how to handle endianness.
UTF-16: Updated unit tests.

- Added wicked unit test to test that only lead/trail surrogate pairs decode, and that any other improper pairing does not. It takes 6 seconds in debug and must be pared down.