Path expressions

PathExpression ->
      PathInExpression
    | QualifiedPathInExpression

A path used as an expression context denotes either a local variable or an item.

Path expressions that resolve to local or static variables are place expressions; other paths are value expressions.

Using a static mut variable requires an unsafe block.

# mod globals {
#     pub static STATIC_VAR: i32 = 5;
#     pub static mut STATIC_MUT_VAR: i32 = 7;
# }
# let local_var = 3;
local_var;
globals::STATIC_VAR;
unsafe { globals::STATIC_MUT_VAR };
let some_constructor = Some::<i32>;
let push_integer = Vec::<i32>::push;
let slice_reverse = <[i32]>::reverse;

Evaluation of associated constants is handled the same way as const blocks.