Rename buffer to scratch in sorting

[LilithHafner]

sort!(x; buffer) => sort!(x; scratch)

It's not too late to rename this again if folks want to. This has never been public, documented, or part of a release or release candidate.

This PR is very similar to #45711

From @StefanKarpinski's comment here: #45222 (comment)

cc those who have previously expressed interest in the name of this thing: @oscardssmith @musm @StefanKarpinski @pcjentsch @longemen3000 @mkitti @jw3126 @halleysfifthinc @nalimilan

[halleysfifthinc]

Rereading the discussion in #45330 , buffer appears to be the most popular name option. One point against scratch is that (to me) the purpose isn't immediately obvious from the name. To be fair, this is probably something that shouldn't/wouldn't be used without first reading the docstring, but buffer (or even scratchspace) are more familiar and/or self-explanatory.

I also just noticed that this isn't documented in the sort docstring. Was that intentional?

[StefanKarpinski]

I quite like this name. Much clearer imo than "buffer"; the main issue I have with "buffer" is that it is often used for input or output vectors. This is neither—it is only used as a temporary scratch space during the sorting operation and I don't think there's a clearer or more concise name that conveys that than scratch.

[jw3126]

(or even scratchspace) are more familiar and/or self-explanatory.

+1 for scratchspace. It is even more self-explanatory and since this is a low level optimization typing a few more characters seems fine to me.

[StefanKarpinski]

I considered that but the name is used internally and having it be longer than necessary is therefore annoying. I'm not sure scratchspace is clearer than scratch since what else is scratch the start of? But sure, scratchspace would also be good.

[bramtayl]

what else is scratch the start of?

A curated list courtesy autocomplete: the scratchard equation, scratcharia, scratchnapped, scratchmen apoo, scratch-o, scratchtasia

[LilithHafner]

I also just noticed that this isn't documented in the sort docstring. Was that intentional?

Yes. It's not part of the public API because it is new so we might want to change it and nobody has requested that it become a part of the public API. Unlike most features, this one does not suffer much from being internal because its main use case is high-level sorting functions such as sort!(::AbstractMatrix).