Improve environment handling performance and cleanup API

[lahma]

Jint.Benchmark.DromaeoBenchmark

Diff	Method	Prepared	Toolchain	FileName	Mean	Error	Allocated
Old	Run	False	Default	dromaeo-3d-cube	22.222 ms	0.2079 ms	6304.91 KB
New		False	Default		19.953 ms (-10%)	0.2049 ms	6304.9 KB (0%)
Old	Run	False	Default	dromaeo-core-eval	4.296 ms	0.0265 ms	327.85 KB
New		False	Default		3.624 ms (-16%)	0.0306 ms	327.84 KB (0%)
Old	Run	False	Default	dromaeo-object-array	42.425 ms	0.1915 ms	100367.72 KB
New		False	Default		41.721 ms (-2%)	0.3537 ms	100367.76 KB (0%)
Old	Run	False	Default	droma(...)egexp [21]	149.899 ms	1.0581 ms	151325.94 KB
New		False	Default		147.555 ms (-2%)	1.1606 ms	151475.38 KB (0%)
Old	Run	False	Default	droma(...)tring [21]	255.347 ms	5.2541 ms	1316020.46 KB
New		False	Default		238.086 ms (-7%)	4.7367 ms	1315806.83 KB (0%)
Old	Run	False	Default	droma(...)ase64 [21]	49.009 ms	0.3184 ms	2367.98 KB
New		False	Default		45.742 ms (-7%)	0.1953 ms	2367.95 KB (0%)
Old	Run	True	Default	dromaeo-3d-cube	21.630 ms	0.0634 ms	5966.65 KB
New		True	Default		18.718 ms (-13%)	0.1213 ms	5966.66 KB (0%)
Old	Run	True	Default	dromaeo-core-eval	4.554 ms	0.0182 ms	312.77 KB
New		True	Default		5.395 ms (+18%)	0.0273 ms	312.78 KB (0%)
Old	Run	True	Default	dromaeo-object-array	45.505 ms	0.2457 ms	100320.31 KB
New		True	Default		41.496 ms (-9%)	0.4877 ms	100320.36 KB (0%)
Old	Run	True	Default	droma(...)egexp [21]	110.251 ms	2.1159 ms	148277.7 KB
New		True	Default		109.215 ms (-1%)	0.9305 ms	152477.6 KB (+3%)
Old	Run	True	Default	droma(...)tring [21]	252.442 ms	5.3163 ms	1315823.89 KB
New		True	Default		239.481 ms (-5%)	5.0117 ms	1315753.4 KB (0%)
Old	Run	True	Default	droma(...)ase64 [21]	45.947 ms	0.2130 ms	2268.82 KB
New		True	Default		42.438 ms (-8%)	0.2702 ms	2268.79 KB (0%)