perf(luaJIT): use LuaJIT stringbuffers, if present. Fixes #67

[kikito]

No description provided.

[kikito]

LuaJIT CI Perf results in master (no stringbuffers):

nil,string,empty,sequence,record,hybrid,recursive,meta,process,complex,big
0.031032,0.153688,0.075373,0.193429,0.3219,0.402339,0.179187,0.229829,0.137413,0.939343,1.742576

LuaJIT CI Perf results on this branch (string buffers active):

nil,string,empty,sequence,record,hybrid,recursive,meta,process,complex,big
0.063725,0.172401,0.095502,0.121064,0.223445,0.233188,0.169171,0.213383,0.159085,0.81574,1.256349

nil	string	empty	sequence	record	hybrid	recursive	meta	process	complex	big
0.031032	0.153688	0.075373	0.193429	0.3219	0.402339	0.179187	0.229829	0.137413	0.939343	1.742576
0.063725	0.172401	0.095502	0.121064	0.223445	0.233188	0.169171	0.213383	0.159085	0.81574	1.256349
-105.35%	-12.18%	-26.71%	37.41%	30.59%	42.04%	5.59%	7.16%	-15.77%	13.16%	27.90%

Overall there's a significant performance increase on all non-trivial scenarios.

[kikito]

Running this on github's CI servers is really not reliable so I'll try to set things locally and run the perf tests several times before merging.

[kikito]

Ran it locally several times and when LuaJIT has stringbuffers I saw a perf increase of ~20% for anything more complicated than an empty table or a single string. Overall, worth it. So merging this.