Small performance optimization to string handling

[lahma]

• no need to call PutValue when target is not an object and value was mutated in-place
• fast path for FromCharCode
• improve ProbablyBlockStatement

Jint.Benchmark.DromaeoBenchmark

Diff	Method	FileName	Mean	Error	Allocated
Old	Run	dromaeo-3d-cube	25.803 ms	0.0178 ms	6247.69 KB
New			25.639 ms (-1%)	0.0151 ms	6248.21 KB (0%)
Old	Run	dromaeo-core-eval	6.258 ms	0.0199 ms	321.58 KB
New			5.597 ms (-11%)	0.0244 ms	321.84 KB (0%)
Old	Run	dromaeo-object-array	72.128 ms	0.1724 ms	101492.07 KB
New			69.330 ms (-4%)	0.2562 ms	101492.03 KB (0%)
Old	Run	droma(...)egexp [21]	231.533 ms	1.5412 ms	172628.85 KB
New			222.238 ms (-4%)	1.9947 ms	174399.84 KB (+1%)
Old	Run	droma(...)tring [21]	431.184 ms	37.0669 ms	1325371.05 KB
New			436.327 ms (+1%)	36.2579 ms	1322129.7 KB (0%)
Old	Run	droma(...)ase64 [21]	71.958 ms	0.1181 ms	7658.61 KB
New			70.192 ms (-2%)	0.3129 ms	6537.35 KB (-15%)