Improve JS array manipulation performance

[classilla]

With https://bugzilla.mozilla.org/show_bug.cgi?id=1348772 adapted to 45,

before

time /home/spectre/Desktop/TenFourFoxDebug.app/Contents/MacOS/js -f tests/basic/shifted-elements4.js
40.491u 0.273s 0:40.73 100.0%   0+0k 0+3io 0pf+0w

after

time ../../../obj-ff-dbg/dist/bin/js -f tests/basic/shifted-elements4.js
0.455u 0.294s 0:00.71 104.2%    0+0k 0+0io 0pf+0w

This is something like an 89x speedup on Array.push(). We should implement the unshifting optimization for the other major ops.

https://bugzilla.mozilla.org/show_bug.cgi?id=1364346 unshift
https://bugzilla.mozilla.org/show_bug.cgi?id=1364345 splice (may or may not need https://bugzilla.mozilla.org/show_bug.cgi?id=1362753 )

[classilla]

For M1364346, I'm concerned about some of the invariants in part 1, so I'm not going to take the fast path optimizations.