Reimplement IO#guaranteeCase and IO.bracketFull

[vasilmkd]

Initially, I was going to say how reimplementing IO#bracketFull in terms of IO#guaranteeCase is clearer and uses less special cased code for guarding user code against misuse of the inherently non-lexical nature of the bracketFull constructor, but it turns out it is both simpler and faster.

series/3.x:

Benchmark               (size)   Mode  Cnt      Score     Error  Units
AsyncBenchmark.bracket     100  thrpt   20  29369.257 ± 492.607  ops/s

This PR:

Benchmark               (size)   Mode  Cnt      Score     Error  Units
AsyncBenchmark.bracket     100  thrpt   20  30321.272 ± 998.223  ops/s

[djspiewak]

Fascinating!