Improve the construction of "length" built-in strings.

[zherczeg]

The "length" property name is the most frequently used built-in string and also frequently created by various hot-paths. New functions are added to improve the speed of the "length" string creation.

15% impovement on access-fannkuch.js, but other tests benefit from it.

Benchmark	Perf (sec)
3d-cube.js	0.965 -> 0.952 : +1.426%
3d-raytrace.js	1.135 -> 1.131 : +0.305%
access-binary-trees.js	0.593 -> 0.589 : +0.648%
access-fannkuch.js	2.805 -> 2.368 : +15.595%
access-nbody.js	1.124 -> 1.070 : +4.834%
bitops-3bit-bits-in-byte.js	0.584 -> 0.582 : +0.395%
bitops-bits-in-byte.js	0.873 -> 0.869 : +0.435%
bitops-bitwise-and.js	1.175 -> 1.143 : +2.762%
bitops-nsieve-bits.js	1.922 -> 1.813 : +5.657%
controlflow-recursive.js	0.401 -> 0.402 : -0.155%
crypto-aes.js	1.206 -> 1.140 : +5.450%
crypto-md5.js	0.765 -> 0.753 : +1.500%
crypto-sha1.js	0.722 -> 0.714 : +1.095%
date-format-tofte.js	0.883 -> 0.876 : +0.735%
date-format-xparb.js	0.485 -> 0.475 : +2.041%
math-cordic.js	1.365 -> 1.351 : +1.072%
math-partial-sums.js	0.755 -> 0.723 : +4.304%
math-spectral-norm.js	0.604 -> 0.599 : +0.937%
string-base64.js	2.153 -> 2.079 : +3.426%
string-fasta.js	1.868 -> 1.814 : +2.881%
Geometric mean:	+2.831%

Binary size unchanged.

[dbatyai]

LGTM

[LaszloLango]

LGTM