[RFC] num: bitwise reflection methods

A bit-manipulation PR, however I still have some doubts:
 * Library. I believe this is a pretty core operation for several algorithms (eg. many checksums), is the "battery-included" philosophy ok for libstd?
 * Naming. Is `reflect()` too generic? That's how the operation is usually called, but may be too broad as API and conflict with reflection/introspection.
 * Lot's of redundancy here: 8, 16, 32 and 64 variants. I tried implementing it more generically, however:
    * The `Bitwise` trait lacks `One`, `Zero` and `NumCast` for this implementation to work. Maybe they should be added anyway?
    * Is it possible to split `impl Bitwise for i8..i64` in multiple files? Most of it is through instrinsics in each module, but ideally `reflect()` could be directly implemented in uint/int_macros abusing `$T`.

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Let Bitwise provide a reflect() method for reversed binary
representation. This is a binary primitive used in several algorithms,
eg. CRC checksums.