core: add ValueSet::len and Record::len (#2152)

This PR adds two new accessor functions that are useful for creating a
structured serde implementation for tracing.

This is a sort of "distilled" version of
#2113, based on the `v0.1.x`
branch.

As it is unlikely that "structured serde" will be 1:1 compatible with
the existing JSON-based `tracing-serde` (or at least - I'm not sure how
to do it in a reasonable amount of effort), these functions will allow
me to make a separate crate to hold me over until breaking formatting
changes are possible in `tracing-serde`.

CC @hawkw, as we've discussed this pretty extensively