Implement signed and unsigned min and max value.

[Robbepop]

In Rust all primitive types provide a min_value and max_value constructor to constructor the minimum representable and maximum representable value respectively. In ApInt this is not directly possible since ApInt itself has no concept of signedness. However, it is possible to achieve a similar effect and provide the same level of functionality by providing constructors that are aware of the signedness.

The proposed API additions are:

• fn signed_min_value(width: BitWidth) -> ApInt

  Returns the smallest possible ApInt instance with the given BitWidth representing a signed value.

• fn unsigned_min_value(width: BitWidth) -> ApInt

  Returns the smallest possible ApInt instance with the given BitWidth representing an unsigned value.

• fn signed_max_value(width: BitWidth) -> ApInt

  Returns the largest possible ApInt instance with the given BitWidth representing a signed value.

• fn unsigned_max_value(width: BitWidth) -> ApInt

  Returns the largest possible ApInt instance with the given BitWidth representing an unsigned value.

Some higher level data structures that are based on ApInt may later use these explicit constructors in an implicit way. E.g. SignedApInt may provide min_value and max_value directly by simply forwarding to its internal ApInt::signed_min_value and ApInt::signed_max_value.

[Robbepop]

Issue implemented in commit 9d86aa8.