The `val` variable is a register value that we need to be able to return
at any time from `decode_iter`. If it happened that a yield was
triggered while processing trailing whitespace the lack of persistance
caused decode to return a term intialized from a random integer value.
Obviously the Erlang VM did not enjoy this.

Thanks to @michalpalka for the report.

Fixes #66

This implements the `use_nil` option as discussed on issue #64. Passing
the atom `use_nil` as an option to both encode and decode will replace
the atom `null` with `nil` when decoding and encode `nil` as `null` when
encoding values.

Fixes #64
Fixes #68

This patch adds initial support for decoding/encoding to/from the new
maps data type.

I'd like to thank Jihyun Yu (yjh0502) for the initial versions of this
work.

This adds a configurable limit on the number of bytes consumed by
the decoder before yielding back to the Erlang VM. This is to avoid the
infamous scheduler collapse issues.

The `jiffy:decode/2` now takes an option `{bytes_per_iter,
pos_integer()}` that controls the yield frequency. The default value is
2048.

This is ground work to allow Jiffy to yield back to the scheduler.
Creating an encoder resource will allow for the necessary state to be
carried across NIF function invocations.

This is ground work to allow Jiffy to yield back to the scheduler.
Creating a decoder resource will allow for the necessary state to be
carried across NIF function invocations.

As it turns out I did not understand the documenation for
load/upgrade/unload correctly. load/upgrade are called conditionally if
there's code in the VM for the NIF. Ie, no code means load is called,
where as if code exists, upgrade is called.

unload is called regardless once per load/unload. This means that
load/upgrade in Jiffy's case will each create a state object and unload
will free it each time. I was missing the fact that unload is called
every time and hence I don't need to clear the state in upgrade.

The docs aren't entirely clear on the order of calls for upgrades so
this is mostly just in case old_priv ever happens to not be what load
returned in priv.

By default Jiffy is quite strict in what it encodes. By default it will
not allow invalid UTF-8 to be produced. This can cause issues when
attempting to encode JSON that was decoded by other libraries as UTF-8
semantics are not uniformly enforced.

This patch adds an option 'force_utf8' to the encoder. If encoding hits
an error for an invalid string it will forcefully mutate the object to
contain only valid UTF-8 and return the resulting encoded JSON.

For the most part this means it will strip any garbage data from
binaries replacing it replacement codepoint U+FFFD. Although, it will
also try and the common error of encoding surrogate pairs as three-byte
sequences and reencode them into UTF-8 properly.

jiffy:encode/2 now accepts a 'pretty' option that will format the JSON
output using two-space indents.

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\"">>

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.

* Refs became atoms to make sure they can live across calls
  to the NIF functions.
* Initialized curr in decode so that I'm no longer pushing
  random values into the Erlang VM.

Passes all eep0018 tests.