compute: avoid needless allocations of strings in text-to-varchar conversion

[mgree]

While working with @frankmcsherry, we noticed that text_to_varchar was allocating a fresh string every time, even though VarChar<S> accepts &str.

Motivation

• This PR fixes a previously unreported bug.

Even when the text fits in the varchar limit, we would allocate a new string. There's no need to do this: just pass a reference.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:

[antiguru]

Looks good +/- suggested changes.

[mgree]

Huh... I could have sworn I compiled it on my machine first. 😅

Main's CI seems to be broken, will rebase once a fix lands.