Luc Préfontaine kindly pointed out that `.writeUTF` and `.readUTF` have a maximum length of
64k. This commit switches to a custom integer-length prefix for strings and reader types to
boost their max length. Keywords are unchanged.
Old string and reader type IDs are now deprecated but will continue to be supported for
thawing.
`write-as-bytes!` was unnecessarily general. Renamed as type-hinted `write-biginteger!`.
Improves write performance of all types that relied on `write-as-bytes`: bigints, bigdecs,
rational.
