Add a with_capacity function

[TethysSvensson]

No description provided.

[fitzgen]

Looks good, but I'd like to add a little more of a smoke test before merging:

Can you add a test that creates a Bump with a funky capacity (something that is not footer aligned, like 11) and then also allocate things out of that bump arena that require more alignment (like u128s)? At least enough to cause the bump arena to allocate a new chunk.

This probably belongs in tests/tests.rs rather than the doc comment.

Thanks!

[TethysSvensson]

Can you add a test that creates a Bump with a funky capacity (something that is not footer aligned, like 11) and then also allocate things out of that bump arena that require more alignment (like u128s)? At least enough to cause the bump arena to allocate a new chunk.

Yes, I can definitely add a test like that. :)

I have also simplified the code a bit, to make it more obvious that this will never allocate less than a normal call to Bump::new() would.

[TethysSvensson]

@fitzgen I've now added the test.

[fitzgen]

Thanks!