Compile with stable Rust

[pickfire]

ve currently uses quite a bit of nightly rust since it is based on liballoc, although some use of nightly is removed, there is still quite a bit of it.

• allocator_api
• allow_internal_unstable
• const_fn
• const_generics
• core_intrinsics
• exact_size_is_empty
• dropck_eyepatch
• new_uninit
• slice_partition_dedup
• slice_ptr_len
• min_specialization
• rustc_attrs
• trusted_len
• try_reserve
• unsafe_block_in_unsafe_fn

CC @maciejhirsz any idea how to reduce the use of nightly features? beef looks quite different, not sure if re-architecting while porting would be a good idea

[maciejhirsz]

Without looking at code here, Cow::const_str is enabled on stable if you need strings only, slices unfortunately require generics to be const proper, (I could maybe add Cow::const_bytes).