Add const-generic bitfield accessors for compile-time optimization

Add a specialized impl block for `__BindgenBitfieldUnit<[u8; N]>` with
const-generic methods: `get_const`, `set_const`, `raw_get_const`, and
`raw_set_const`. These methods take bit offset and width as const generic
parameters, allowing the compiler to fully optimize bitfield access at
compile time.

Update codegen to use the new const-generic methods for generated
bitfield accessors. Since all bitfield offsets and widths are known at
code generation time, this enables better optimization.

The original runtime-parameter methods are preserved for backwards
compatibility with users who provide custom implementations via the
blocklist mechanism.

Author: Jeremy Fitzhardinge