Updating IsNA trait

[Ilia-Kosenkov]

Trait IsNA is modified and renamed toCanBeNA.
New static function (i.e. function with no self arg) is introduced::
fn CanBeNA::na() -> Self
which can be used to obtain NA value of a given type.

Modified code in key places to switch to <type>::na() instead of helper functions.

I did not remove implementations of CanBeNA for primitive types (like f64 or i32).
This may be deprecated in the future.

[andy-thomason]

Nice. Good to have a type-specific NA generator.

You can probably lose the turbofish <&std>:: in many cases as the type inference may do
this for you.

[Ilia-Kosenkov]

@andy-thomason,
Actually, it does not seem to work. &str::na(), for instance, is treated as &(str::na()), and there is no CanBeNA implementation for str. I, however, removed unnecessary <> for non-reference types like f64, but only in NA-related code.

Perhaps I am missing some Rust syntax feature.

[Ilia-Kosenkov]

CI fails due to 404 while getting R-devel for mac. I hate that whenever there is a version bump (or some other update) on CRAN side, it is always a wild 404 on one of the platforms.
Will re-run the CI later.