Construct the underlying map with_capacity

[apasel422]

Since the capacity is restricted, it makes sense not to over- or underallocate on creation.

[Gankra]

One potential argument against this is that you might never get to the limit.

[apasel422]

In that case, LruCache should expose a way to set the maximum size separately from the maximum storage capacity.

[Gankra]

s/maximum/minimum, but yeah that wouldn't be unreasonable.

[jstnlef]

I should be able to bang something out for this in the next hour or so if that's alright.

[apasel422]

@jstnlef Go for it!