Name bin target differently from lib

[Mark-Simulacrum]

This avoids file collisions, which lead to spurious rebuilds and errors from
Cargo (recently added by rust-lang/cargo#10172).

I haven't been able to confirm this locally yet, possibly because the wrong
Cargo gets used (not sure), but this should hopefully fix the benchmark on perf.r-l.o.

[Mark-Simulacrum]

This avoids clobbering the output (good, I guess) but still runs into the same problem. Discussing on https://rust-lang.zulipchat.com/#narrow/stream/246057-t-cargo/topic/document.20lib.20before.20bin.

[Mark-Simulacrum]

rust-lang/cargo#10201 should let us move forward here once a Cargo bump including it lands on rust-lang/rust.