[Variant] Simplify Builder buffer operations (#7795)

# Rationale for this change

This PR simplifies how we build up the internal `VariantBuffer` by
appending to it directly, rather than pre-allocating a buffer filled
with zeroes and setting values at indices. This avoids indexing math
that can be hard to follow and reason about.

This PR also aims to design a well-defined API for `ValueBuffer`. My
thought here was we should not touch the inner `Vec<u8>`. It's quite
sensitive.

# Are there any user-facing changes?

Nope!

Author: friendlymatthew