[#717] Handle oddly-sized integral types with the int-cast facilities

Description

We currently have Mutez, TLTime integral types with odd sizes, that we treat in a special way and define specific conversion functions for. We should stop this special treatment and handle Mutez and TLTime as any other integral type.

This defines the right instances of Bits, Integral, IntBaseType and bring in int-cast facilities for Mutez and TLTime.

Related issue(s)

Resolves #717 (closed)

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