Add padded allocator for aligned and padded vectors. (#460)

Padded vectors with run-time padding/alignment guarantees will form the basis of the storage class for the new CPU and SIMD generated mechanisms.

* Add `padded_allocator` that aligns and pads allocations.
* Make microbenchmark for `default_construct_adaptor` that overrides the allocator construct() to default- instead of value-initialization on values.
* Add `with_instrumented_malloc` class for tracking malloc, realloc, etc. calls.
* Add unit tests for `padded_allocator`.