Changes

Minor heap reservation: change the terminology

The current codebase use 'caml_minor_heaps_{start,end}' for the
boundaries of a global address space that is reserved,
'dom->caml_minor_heap_area_{start,end}' for a 'minor heap area', a
segment of this address space that is owned by each domain, and then
finally 'dom->young_{start,end}' for the prefix of this segment that
is actually committed and used as the minor heap of each domain. Some
comments refer to the latter as the 'minor heap arena', following
terminology from the Retrofitting Parallelism into OCaml paper.

On a suggestion by KC, I am trying to make the naming scheme more
regular by consistently using 'reservation' for a reserved block of
address space:

- Use 'minor heaps reservation' for the global reservation. Its
  boundaries remain stored in 'caml_minor_heaps_{start,end}' to avoid
  compatibility issues in third-party code.

- Use 'minor heap reservation' for the per-domain segment of the
  global reservation. Its boundaries are stored in
  'dom->minor_heap_reservation_{start,end}'.

- Use 'minor heap' for the prefix of the minor heap reservation
  that is actually committed, whose boundaries remain
  'dom->young_{start,end}'.

This change is purely local to domain.c.

Suggested-by: KC Sivaramakrishnan <kc@kcsrk.info>
 (commit: 08d94e7)

review: use 'arena' for the (committed) minor heap space

Suggested-by: KC Sivaramakrishnan <kc@kcsrk.info>
 (commit: 0fd3dac)

self-review: solidify two separate comments on minor heap layout
 (commit: 2e0836f)