This patch significantly speeds up parsing of small BigInts. Its
primary idea is to move the loop that's iterating over the string
into the FromStringAccumulator API. That enables using function-
local variables instead of member fields.
A second optimization is to use a stack-allocated digit_t[] array
for small sizes, before falling back to a (comparatively slow)
std::vector.
As a particularly fast path, when this stack-allocated storage is
guaranteed to be enough, we can perform inlined multiply-and-add
steps directly on that data.
Finally, this patch changes the conversion of characters to their
numeric values from computations to a lookup table, which is a bit
faster for radixes <= 10 (where, in the old code, only one range
needed to be checked), and a lot faster for radixes > 10.

Bug: v8:11515
Change-Id: Ifd8ec4799ac34447ba6d4350b7788b559307784c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3064603
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76210}