Add FixedBitset::ones_with_capacity() to create all-ones bitset efficiently

[hniksic]

Fixes #101

This PR includes:

• the new FixedBitset::ones_with_capacity() constructor
• optimization to with_capacity_and_blocks() to avoid double initialization
• minor warning fixes

Compared to the last version in #101, this implementation of with_capacity_and_blocks() makes sure to mask off unused bits in the last block to ensure == and Hash work correctly.

[hniksic]

I've now noticed that this PR, as originally submitted, subtly changes the behavior of with_capacity_and_blocks(): it masks off bits in the last block that exceed the capacity. The original implementation stored those bits as provided by the caller. While one could argue this is a bug fix (the docs say "truncated to the capacity"), it's also a change to a public API that existing code might rely on.

To avoid this, I've added a commit that moves the masking logic into ones_with_capacity() itself, which generates the correctly masked last block value directly rather than relying on with_capacity_and_blocks() to clean it up. I've also updated docs of with_capacity_and_blocks() to clarify current behavior.