Clarify HashMap's capacity handling.
HashMap has two notions of "capacity":
- "Usable capacity": the number of elements a hash map can hold without
resizing. This is the meaning of "capacity" used in HashMap's API,
e.g. the `with_capacity()` function.
- "Internal capacity": the number of allocated slots. Except for the
zero case, it is always larger than the usable capacity (because some
slots must be left empty) and is always a power of two.
HashMap's code is confusing because it does a poor job of
distinguishing these two meanings. I propose using two different terms
for these two concepts. Because "capacity" is already used in HashMap's
API to mean "usable capacity", I will use a different word for "internal
capacity". I propose "span", though I'm happy to consider other names.
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