The `FAST_` prefix doesn’t make much sense — they’re all just different cases
with their own optimizations. Packedness being implicit (e.g. `FAST_ELEMENTS`
vs. `FAST_HOLEY_ELEMENTS`) is not ideal, either.

This patch renames the FAST elements kinds as follows:

- e.g. FAST_ELEMENTS => PACKED_ELEMENTS
- e.g. FAST_HOLEY_ELEMENTS => HOLEY_ELEMENTS

The following exceptions are left intact, for lack of a better name:

- FAST_SLOPPY_ARGUMENTS_ELEMENTS
- SLOW_SLOPPY_ARGUMENTS_ELEMENTS
- FAST_STRING_WRAPPER_ELEMENTS
- SLOW_STRING_WRAPPER_ELEMENTS

This makes it easier to reason about elements kinds, and less confusing to
explain how they’re used.

R=jkummerow@chromium.org, cbruni@chromium.org
BUG=v8:6548

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ie7c6bee85583c3d84b730f7aebbd70c1efa38af9
Reviewed-on: https://chromium-review.googlesource.com/556032Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46361}