fix SimpleString equality for content with embedded NUL bytes

[aysha-afrah26]

On master, with the regression test from this branch:

gtest_simple_string.cpp:387: Failure
Value of: s1 == s2
  Actual: true
Expected: false

s1 and s2 there are "a\0b" and "a\0c": operator== and operator!= compare
with strncmp, which stops at the first NUL byte, while operator< compares the
full length with memcmp, so the same pair is reported both equal and ordered.
Content with embedded NUL bytes is not hypothetical for Any, since
ImportBlackboardFromJSON decodes a JSON \u0000 escape into a real NUL and the
scripting == / != route through SimpleString: restoring
{"a":"p\u0000X","b":"p\u0000Y"} into a tree whose only node is
<AlwaysFailure _skipIf="a == b"/> ticks to SKIPPED, so the guard skips a node over
two values that are not equal. Switching the two operators to memcmp is enough,
and the size check in front of it keeps the read in bounds.

Full ctest is 526/526 on the macOS exclusion set, and the new test covers both the
in-place and the heap representation.