Cleaned, documented, wrote tests for up std::bool
Removed unused import warning in std::mem and cleaned it up too

Removed is_true and is_false from std::bool

Removed freestanding functions in std::bool
auto merge of #10007 : Kimundi/rust/add_docs_01, r=cmr

Additionally:
- Sorted the prelude reexports
- Removed unused import warning in std::mem and cleaned it up too

(I was bored :ghost: )