- Error if trailing characters are present in both JSON and msgpack
decoders.
- Fix a bug in utf-16 codepoint decoding
After these changes, the JSON decoder fully passes the JSONTestSuite
tests at https://github.com/nst/JSONTestSuite.

We now track the path to the current object in the deserializer,
and display the faulty path in the error message.

Track start, end, and position pointers instead of start + read index.
This was already done in the JSON decoder (where a position pointer was
a bit easier to work with), we know do this in the msgpack decoder as
well.

We were using a mix of `js_`/`json_` and `mp_`/`mpack_` prefixes.
Standardize on the longer prefixes.

- Remove need for `TypeNode_Repr` - we now rely on the builtin reprs
- Add module name to decoder reprs

Datetimes are encoded as RFC 3339 formatted strings.

Previously we attempted to support serializing both naive and aware
datetime objects. Since MsgPack timestamps are by nature "aware", we
interpreted naive datetimes implicitly as representing local time. This
assumption could be a footgun for unaware users, and so is removed. Only
aware datetimes are supported.
This commit also dramatically improves the performance of msgpack
datetime encoding/decoding (which previously relied on calling into
python methods to do the conversion). datetime <-> epoch conversion is
now handled entirely within msgspec itself, resulting in ~10x speedup on
encoding and ~5x speedup on decoding.

Previously for JSON datetimes with a timezone offset, we'd decode the
datetime into the specified timezone. To avoid the cost of creating a
new `datetime.timezone` object every time, we'd cache the most recent
timezone to reuse it if possible. However, the msgpack decoder always
decodes datetimes into UTC, and most users will probably want UTC
anyway. To keep things compatible, we now always decode all datetimes
into UTC for JSON, applying the offset as needed. An added perk is that
this is much faster, as we don't ever hit Python method calls on decode.

These are tests for valid/invalid JSON, not correctness of parsing.

Also fixup a few bugs in the float parsing routines.

Thank goodness for fuzz testing.

Not all types are JSON compatible (e.g. dicts with non-str keys). We
previously attempted to handle this, but it didn't cover all cases. We
now improve this error handling, and add test cases for this behavior.

Add tests for datetimes and structs

Still need to redo the prose docs, but in a later PR

Still need to redo the full docs, but this at least is a start.