Resolve arith overflow on with_capacity

[HeeillWang]

resolve #626 #627

[hawkw]

this looks good to me overall, thanks for the fix!

i had some minor suggestions for improving the panic messages so that the requested capacity is displayed to the user. what do you think?

[WhyNotHugo]

Function docstring should clarify this function panics:

/// # Panics
/// 
/// Requested capacity too large: would overflow `usize`.

[HeeillWang]

Function docstring should clarify this function panics:

/// # Panics
/// 
/// Requested capacity too large: would overflow `usize`.

reflected!

[WhyNotHugo]

TBH, this could just be:

n.checked_add(n / 3).expect("requested capacity must fit usize without overflow")

The only downside if that the requested capacity is not shown in the error, so maybe you'll want to hear what the maintainers think before applying any changes.

(the same applies to the above match that basically hand-rolls an expect).