Add Vec::resize_with

[jschillem]

Closes #648.

Adds Vec::resize_with, the closure-based counterpart to the existing resize method, in line with the std library resize_with method.

This method is valuable when you wish to resize the Vec and require some computation for each new element.

Changes

• resize_with method added to Vec with documentation specifying when to use each of the resize functions.
• Test coverage matching the resize method's coverage, as well as coverage for per-slot closure invocation, to test the invariant mentioned in the documentation.