support $x variable interpolation

[stevengj]

It would be nice if the @b macro supported $x interpolation of globals and other expressions into benchmarked expressions, to make it easier to benchmark expressions using global data without using @eval and without writing a function.

This is a widely used feature of BenchmarkTools / @btime … why not copy it?

[LilithHafner]

I don't know how to do interpolation without memory leaks. 😿

Explanation in the docs: https://chairmarks.lilithhafner.com/stable/migration/#footnote-1
Julia issue at the root of this: JuliaLang/julia#14495
Corresponding issue in BenchmarkTools.jl: JuliaCI/BenchmarkTools.jl#339

In theory it would be possible to support interpolation with memory leaks & non-interpolation without memory leaks. I'll look into that.

[LilithHafner]

Okay, I think I figured it out. PR forthcoming 🐱