Add benchmark for deeply nested types

[ishitatsuyuki]

Request per rust-lang/rust#48296 (comment).

Test case taken from rust-lang/rust#38528 (comment).

[Mark-Simulacrum]

Hm, I wonder if we could name this something like deeply-nested and then add a note to the Cargo.toml about which issue it specifically addresses -- that would help clarify what's going on at a cursory look at perf.rlo if there's regressions here I think.

Otherwise though this looks good.

[ishitatsuyuki]

r? @Mark-Simulacrum

[Mark-Simulacrum]

Looks good to me. I'll deploy soon.