Expose the low-level encoding/decoding primitives in a sub package

[burdiyan]

Sometimes it can be quite useful to be able to manually write some data into a buffer, be it for performance reasons, or for some obscure encoding customizations needs.

Currently the low-level encoding/decoding functions are unexported in this module.

I suggest exposing the low-level encoding primitives as a separate package in the same module, similar to how the newly proposed v2 of the JSON package os the standard library exposes the similar primitives in its jsontext package.