Scrub allocator code

[malachib]

Code works, but is super crusty. Rework:

• Elevate usage of new allocator_options paradigm
  • With handle_descriptor and handle_descriptor_base
• single_fixedbuf_runtimesize_allocator::InitParam ought to go
• Consolidate estd::internal::policy_base and allocator_options
• Consider using regular old iterators (pointers) instead of locking_iterator, locking_accessor, etc. Note though we don't want to throw out dogfooding, despite its minimal usage locking is near to our hearts still (see pgm_string)

Overlaps with #88 and #97