Replace rustc_layout_scalar_valid_range_start attribute with pattern types

[oli-obk]

the rustc_layout_scalar_valid_range_start and rustc_layout_scalar_valid_range_end attributes require a lot of machinery to stay sanely usable. Pattern types don't require less machinery, but they are simpler as they don't break through layers of layouts and then require surprisingly subtle unsafe code to be used correctly.

So we're ripping out the attributes, and replace their usages in libcore and rustc:

• std::ptr::NonNull
• std::num::NonZero
• rustc_index_macros::newtype::NewType
• std::num::Nanoseconds

cc @Veykril @scottmcm

[scottmcm]

Very cool! I'm excited to hear these are far enough along that there's a subset we can stabilize enable without incomplete_features in libcore with confidence.

EDIT: Changed "stabilize" because c-e is 100% correct about not stable.

[compiler-errors]

we can stabilize in libcore with confidence.

Well, I think it's very important to note here that we are not stabilizing anything. We're just changing the underlying representation of the struct to use different machinery. We should continue to be able to remove all pattern types from the compiler and standard library and go back to using rustc_* attrs at any point in this experimentation.