Create ExpressionCache to unify constant lookups

[lahma]

Puts common logic and duplicated code to one place.

[lahma]

No major differences in performance.

Jint.Benchmark.DromaeoBenchmark

Diff	Method	FileName	Mean	Error	Allocated
Old	Run	dromaeo-3d-cube	15.778 ms	0.2194 ms	6303.53 KB
New			15.480 ms (-2%)	0.1574 ms	6305.56 KB (0%)
Old	Run	dromaeo-core-eval	3.090 ms	0.0325 ms	327.5 KB
New			3.002 ms (-3%)	0.0252 ms	327.42 KB (0%)
Old	Run	dromaeo-object-array	31.589 ms	0.4030 ms	96259.27 KB
New			32.026 ms (+1%)	0.2861 ms	96259.08 KB (0%)
Old	Run	droma(...)egexp [21]	125.476 ms	2.5058 ms	150032.96 KB
New			121.780 ms (-3%)	2.3399 ms	150597.65 KB (0%)
Old	Run	droma(...)tring [21]	239.919 ms	5.4312 ms	1315983.34 KB
New			233.933 ms (-2%)	4.9587 ms	1315737.61 KB (0%)
Old	Run	droma(...)ase64 [21]	35.003 ms	0.5498 ms	2367.54 KB
New			35.679 ms (+2%)	0.6749 ms	2367.42 KB (0%)
Old	Run	dromaeo-3d-cube	15.177 ms	0.1215 ms	5965.28 KB
New			15.202 ms (0%)	0.1546 ms	5967.31 KB (0%)
Old	Run	dromaeo-core-eval	3.032 ms	0.0233 ms	312.43 KB
New			3.009 ms (-1%)	0.0307 ms	312.35 KB (0%)
Old	Run	dromaeo-object-array	32.452 ms	0.4877 ms	96211.86 KB
New			32.070 ms (-1%)	0.2978 ms	96211.69 KB (0%)
Old	Run	droma(...)egexp [21]	93.176 ms	1.8621 ms	152871.14 KB
New			92.373 ms (-1%)	1.8454 ms	152179.04 KB (0%)
Old	Run	droma(...)tring [21]	241.801 ms	8.5015 ms	1315758.23 KB
New			237.291 ms (-2%)	9.9897 ms	1315752.98 KB (0%)
Old	Run	droma(...)ase64 [21]	35.643 ms	0.5987 ms	2268.38 KB
New			33.913 ms (-5%)	0.6555 ms	2268.24 KB (0%)

Jint.Benchmark.SunSpiderBenchmark

Diff	Method	FileName	Mean	Error	Allocated
Old	Run	3d-cube	125.04 ms	1.457 ms	45164.77 KB
New			122.41 ms (-2%)	1.492 ms	45166.97 KB (0%)
Old	Run	3d-morph	109.38 ms	1.025 ms	46258.08 KB
New			110.58 ms (+1%)	0.773 ms	46258.12 KB (0%)
Old	Run	3d-raytrace	118.38 ms	1.454 ms	86418.77 KB
New			115.00 ms (-3%)	0.895 ms	86419.7 KB (0%)
Old	Run	access-binary-trees	65.11 ms	0.838 ms	62249.23 KB
New			64.55 ms (-1%)	0.982 ms	62249.15 KB (0%)
Old	Run	access-fannkuch	271.38 ms	2.336 ms	103.84 KB
New			281.50 ms (+4%)	5.519 ms	103.82 KB (0%)
Old	Run	access-nbody	126.09 ms	0.829 ms	53295.3 KB
New			132.79 ms (+5%)	1.889 ms	53295.29 KB (0%)
Old	Run	access-nsieve	89.84 ms	0.730 ms	17142.67 KB
New			92.39 ms (+3%)	1.201 ms	17142.64 KB (0%)
Old	Run	bitop(...)-byte [24]	96.59 ms	1.071 ms	61948.44 KB
New			100.41 ms (+4%)	1.883 ms	61948.44 KB (0%)
Old	Run	bitops-bits-in-byte	147.83 ms	2.501 ms	40544.59 KB
New			148.46 ms (0%)	1.495 ms	40544.67 KB (0%)
Old	Run	bitops-bitwise-and	83.29 ms	0.607 ms	55938.95 KB
New			82.73 ms (-1%)	0.935 ms	55939.01 KB (0%)
Old	Run	bitops-nsieve-bits	135.38 ms	1.805 ms	53916.36 KB
New			137.89 ms (+2%)	1.576 ms	53916.37 KB (0%)
Old	Run	contr(...)rsive [21]	75.75 ms	0.938 ms	92772.97 KB
New			76.70 ms (+1%)	1.207 ms	92772.97 KB (0%)
Old	Run	crypto-aes	80.93 ms	0.835 ms	10749.68 KB
New			82.92 ms (+2%)	1.135 ms	10731.43 KB (0%)
Old	Run	crypto-md5	70.33 ms	0.915 ms	82176.43 KB
New			71.67 ms (+2%)	0.974 ms	82176.42 KB (0%)
Old	Run	crypto-sha1	68.12 ms	0.613 ms	68861.22 KB
New			70.07 ms (+3%)	0.335 ms	68861.17 KB (0%)
Old	Run	date-format-tofte	66.77 ms	1.147 ms	54569.06 KB
New			67.45 ms (+1%)	0.720 ms	54161.32 KB (-1%)
Old	Run	date-format-xparb	38.01 ms	0.755 ms	26400.13 KB
New			38.31 ms (+1%)	0.758 ms	26400.21 KB (0%)
Old	Run	math-cordic	217.84 ms	3.686 ms	86859.77 KB
New			216.07 ms (-1%)	3.070 ms	86859.41 KB (0%)
Old	Run	math-partial-sums	77.78 ms	0.975 ms	49367.85 KB
New			77.27 ms (-1%)	0.703 ms	49367.9 KB (0%)
Old	Run	math-spectral-norm	80.72 ms	1.310 ms	56617.52 KB
New			81.59 ms (+1%)	1.179 ms	56617.52 KB (0%)
Old	Run	regexp-dna	100.39 ms	1.011 ms	16824.85 KB
New			101.04 ms (+1%)	1.529 ms	16829.54 KB (0%)
Old	Run	string-base64	52.29 ms	0.432 ms	3156.24 KB
New			51.43 ms (-2%)	0.657 ms	3156.21 KB (0%)
Old	Run	string-fasta	118.04 ms	2.052 ms	104786.09 KB
New			111.50 ms (-6%)	1.019 ms	104785.97 KB (0%)
Old	Run	string-tagcloud	53.91 ms	0.788 ms	42042.57 KB
New			53.61 ms (-1%)	0.734 ms	42218.87 KB (0%)
Old	Run	string-unpack-code	52.66 ms	0.787 ms	73810.69 KB
New			50.69 ms (-4%)	0.662 ms	73810.13 KB (0%)
Old	Run	strin(...)input [21]	43.06 ms	0.581 ms	20687.73 KB
New			42.64 ms (-1%)	0.365 ms	20687.13 KB (0%)