Rename MutexArc access methods to unsafe_access
Add a safe implementation of MutexArc::access* methods

Current access methods are nestable and unsafe. This patch renames
current methods implementation - prepends unsafe_ - and implements 2 new
methods that are both safe and un-nestable.

Fixes #7473
Fixed docs and styles
Use MuextArc and RWArc in docstrings
auto merge of #8966 : FlaPer87/rust/issue/7473, r=bblum

Current access methods are nestable and unsafe. This patch renames
current methods implementation - prepends unsafe_ - and implements 2 new
methods that are both safe and un-nestable.

Fixes #7473