It was possible to pass some types of invalid UTF-8 through Jiffy's
encoder. Specifically, if uescaping isn't used, values that would decode
from 0xD800 to 0xDFFFF, 0xFFFE, 0xFFFF, and values greater than 0x10FFFF
would not be flagged as invalid. Now they are.

The encoding length for unicode values was applied to the wrong index
variable.

Thanks to Paul Guyot for the detailed bug report and fix.

Fixes #7

I was accidentally truncating numbers when casting to long on
machines that don't have longs that are 8 bytes.

Fixes #5

The encoder can now return \u escaped unicode data instead of leaving
it as UTF-8 byte sequences. This done like so:

    Eshell V5.8.3  (abort with ^G)
    1> jiffy:encode(<<240, 144, 129, 128>>, [uescape]).
    <<"\"\\uD800\\uDC40\"">>

The encode and decode functions now return the value directly without
being wrapped in a tuple on success. If there is an error, it is
thrown. This is to more closely match the semantics of term_to_binary
and binary_to_term.

Feed the lawyers.

Any number that can't be decoded in C is now passed back
to Erlang for decoding.

Large numbers passed to the encoder will make it through
and be processed in Erlang after the main encoding
process.

Passes all eep0018 tests.