add `HexUint`

See the `HexUint` doc comments.

Additional changes:

`NullTime.MarshalJSON` once again marshals nanoseconds.
This fixes a small bug introduced in the previous commit.

Avoid spurious heap escapes in `ReadUuid` and `ReadNullUuid`.

avoid overheads in JSON encoding

This commit fixes some easily avoidable overheads in `.MarshalJSON`
for the types listed below, resulting in faster encoding due to less
copying and lower allocator usage.

* `Interval`
* `NullFloat`
* `NullInt`
* `NullInterval`
* `NullString`
* `NullTime`
* `NullUint`
* `NullUrl` (also improves `.AppendTo`)

fix `Url.AddPath`; other minor changes; split tests

* `Url.AddPath` now ensures structural consistency between URLs
  generated by calling this method and equivalent URLs decoded
  from text.

* Breaking change in `Join`. Previously, as a special case, it
  accepted a single empty argument. Now, any empty argument is
  rejected with a panic, regardless of argument count.

* `Ter.BoolPtr` now always allocates a new boolean pointer to
  ensure no accidental shared mutations.

add `NullTime.SubInterval`, `NullTime.SubNullInterval`

add `NullUrl.UrlPtr`; safer `NullUrl.Url`

rename `.Append` to `.AppendTo`

add `NullUrl.AddPath`

simpler behavior of `NullTime` less/more functions

`NullTime` also supports parsing Unix millisecond timestamps

Tentative. Allows easy compatibility with JS timestamps.

various minor tweaks and fixes