Renamed ast::Purity to ast::FnStyle and ast::ImpureFn to ast::NormalFn and updated associated variable and function names.
auto merge of #13371 : kaseyc/rust/Rename_Purity, r=brson

Renamed ast::Purity to ast::FnStyle and ast::ImpureFn to ast::NormalFn.
Additionally updated associated variable and function names and test cases.
Fixes #7287