Fix toString as '[Object object]' in a vm

[dcousens]

Builds on #255, thanks to @markdalgleish for the base code.
This pull request shortcuts the toString.toString() workaround by ensuring it fails the existing arg.toString !== Object.prototype.toString first.

Seeing as toString is an edge case in the first place, this balances our performance by having low to no impact in typical usage.
We have the option to shortcut with hasOwnProperty('toString') if that code path is desired, but I have opted out of that for now.

Benchmark results for this pull request

Mozilla/5.0 (X11; Linux x86_64; rv:104.0) Gecko/20100101 Firefox/104.0
* local#strings x 9,997,500 ops/sec ±1.21% (64 runs sampled)
* npm#strings x 10,106,445 ops/sec ±0.79% (66 runs sampled)
* local/dedupe#strings x 2,056,185 ops/sec ±0.34% (67 runs sampled)
* npm/dedupe#strings x 2,144,413 ops/sec ±0.25% (68 runs sampled)

> Fastest is npm#strings | local#strings

* local#object x 6,763,187 ops/sec ±0.80% (67 runs sampled)
* npm#object x 6,685,687 ops/sec ±0.91% (67 runs sampled)
* local/dedupe#object x 3,986,428 ops/sec ±1.21% (38 runs sampled)
* npm/dedupe#object x 3,955,846 ops/sec ±0.54% (66 runs sampled)

> Fastest is local#object | npm#object

* local#strings, object x 6,075,125 ops/sec ±0.94% (66 runs sampled)
* npm#strings, object x 5,996,929 ops/sec ±1.22% (64 runs sampled)
* local/dedupe#strings, object x 2,003,171 ops/sec ±0.64% (67 runs sampled)
* npm/dedupe#strings, object x 2,007,377 ops/sec ±0.43% (67 runs sampled)

> Fastest is local#strings, object | npm#strings, object

* local#mix x 3,123,072 ops/sec ±0.60% (65 runs sampled)
* npm#mix x 3,178,872 ops/sec ±0.69% (66 runs sampled)
* local/dedupe#mix x 817,797 ops/sec ±6.46% (62 runs sampled)
* npm/dedupe#mix x 821,086 ops/sec ±4.42% (64 runs sampled)

> Fastest is npm#mix

* local#arrays x 1,199,652 ops/sec ±0.97% (63 runs sampled)
* npm#arrays x 1,150,411 ops/sec ±2.05% (62 runs sampled)
* local/dedupe#arrays x 716,685 ops/sec ±1.22% (65 runs sampled)
* npm/dedupe#arrays x 755,361 ops/sec ±0.39% (67 runs sampled)

> Fastest is local#arrays

Finished

[julienw]

This variable seems unused :-)
(not a big deal)