wasm-macro-gen.h is mainly used from tests, but LocalDeclEncoder is
also used from various other places.
This CL moves the LocalDeclEncoder to an own compilation unit. We want
to later move wasm-macro-gen.h to the tests folder.
It also refactors the LocalDeclEncoder to reuse the
LEBHelper::write_u32v and LEBHelper::sizeof_u32v methods instead of
reimplementing it.

R=ahaas@chromium.org

Change-Id: Ia4651436f0544578da7c1c43596d343571942e97
Reviewed-on: https://chromium-review.googlesource.com/486724Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44838}