Add missing bitwise cast

[JohanMabille]

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Some bitwise casts functions (from and to unsigned integers) are missing.

[chriselrod]

What is the API for the cast functions? They don't seem to be documented.

Conversions between integers is missing, eg: https://github.com/QuantStack/xsimd/blob/ca75511f497bc8eb178c42db94d3da4d80a8f318/include/xsimd/types/xsimd_avx_conversion.hpp https://github.com/QuantStack/xsimd/blob/ca75511f497bc8eb178c42db94d3da4d80a8f318/include/xsimd/types/xsimd_avx512_conversion.hpp

Casting between 64 bit and 32 bit unsigned integers is useful for random number generators, for example.

[serge-sans-paille]

Since 1e5e9dc95a9a34c7f54fda61b05c9e925b37f183, cast functions are now documented in the official online doc: https://xsimd.readthedocs.io/en/latest/api/cast_index.html