The old name was a legacy of v2 era, where all headers were stitched
into one. With v3 using separate headers, it is better when they have
proper name.
`SizeIs` can accept both `size_t` and a matcher. In the first case,
it checks whether the size of the range is equal to specified size.
In the second case, it checks whether the provided matcher accepts
the size of the range.
