Implement utility functions count_{zeros|ones}, {leading|trailing}_zeros

[Robbepop]

In Rust all primitive types have methods count_zeros, count_ones, leading_zeros and trailing_zeros. So create better parity with ApInt we should implement those small utility functions. This also helps for implementing higher level data structures later on, such as SignedApInt etc.

This issue proposes the addition of the following methods

• [ ] fn count_zeros(&self) -> usize

  Counts all zero bits in the ApInt.

• [ ] fn count_ones(&self) -> usize

  Counts all one bits in the ApInt.

• [ ] fn leading_zeros(&self) -> usize

  Counts all zero bits up to the first one bit in the ApInt.

• [ ] fn trailing_zeros(&self) -> usize

  Counts all zero bits after the last one bit in the ApInt.

Open Questions

• Where are those new methods defined?

  Since these methods operate mostly on the bit-level representation of the ApInt the submodule that currently fits best is the bitwise submodule within the apint module.

• What means "first" and "last" in the context of leading_zeros and trailing_zeros?

  In Rust "first" stands for most-significant-bit and "last" for least-significant-bit.

[Robbepop]

Implementation finalized with commit 808d12da63263d50b0243dd6e6dacde3d1d88d78.