[Zucchini] Refactor: Dedup Disassembler Make(); simplify BufferRegion.

This CL performs two sets of cleanups, to prepare for Zucchini-match.

(1) Replace duplicate Make() functions in architecture-specific
Disassembler classes with a single template static function:
Disassembler::Make().

(2) Make Element inherit from BufferRegion (and merely add |exe_type|)
so code can be better shared. Details:
- |length| is replaced with |size|.
- Constructor cleanup and order change (impacts man callers).
- Move Element::FitsIn() to BufferRegion; have it used by
  BufferViewBase::covers().

Bug: 778316
Change-Id: I4dd805f43ca45d6d5e9becda466f89c3d3ccbb3a
Reviewed-on: https://chromium-review.googlesource.com/781180


Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518279}