Optimize __lookup function

[chrispcampbell]

The __lookup function is called thousands of times (167548 times, to be exact) when running the En-ROADS model, so some simple optimizations will help improve performance.

I was able to get some decent wins just by reducing the number of instructions in the inner loop. Using binary search could potentially help a bit, but my initial experiment with that didn't provide much improvement over a straight linear search. About half of the lookup tables in En-ROADS are 100-200 elements, while the other half have only 10-50 elements, so linear search is probably sufficient.

[chrispcampbell]

Here are some performance numbers for this change relative to other recent optimizations.

Performance

MacBook Pro (2019) | 2.4 GHz 8-core i9, 32 GB RAM, macOS 10.15

Issue	C run (ms)	Wasm run (ms)	Wasm init (ms)	JS mem (MB)	Page mem (MB)
baseline	45.8	87.5	38.0	94	685
SDE 18	46.0	85.6	18.0	39	672
SDE 19	42.8	49.4	15.0	38	25
SDE 22	34.8	44.8	15.0	38	21
SDE 23	32.7	42.8	13.0	38	32
SDE 24	26.6	38.2	13.0	39	26

iPhone 8 | A11, iOS 13

Issue	C run (ms)	Wasm run (ms)	Wasm init (ms)	JS mem (MB)	Page mem (MB)
baseline	39.9	187.0	165.0	39	645
SDE 18	40.3	219.0	86.0	38	724
SDE 19	40.1	81.6	83.0	38	41
SDE 22	35.5	74.6	86.0	40	40
SDE 23	31.1	73.6	82.0	41	39
SDE 24	28.5	71.6	82.0	40	38

iPad Air (2013) | A7, iOS 12

Issue	C run (ms)	Wasm run (ms)	Wasm init (ms)	JS mem (MB)	Page mem (MB)
baseline	151.0	1372.2	30146.0	77	331
SDE 18	166.0	1408.0	4416.0	42	395
SDE 19	151.0	837.6	1291.0	45	41
SDE 22	137.0	771.6	1484.0	44	40
SDE 23	110.1	642.2	1148.0	44	41
SDE 24	111.8	638.4	1236.0	45	37

Size

Issue	Wasm size (bytes)
baseline	1,084,036
SDE 18	773,968
SDE 19	776,851
SDE 22	737,028
SDE 23	741,668
SDE 24	741,677

Legend

Issue	Date	Notes
baseline	2020/07/08	baseline prior to performance work
SDE 18	2020/07/09	change lookup init to use static arrays
SDE 19	2020/07/09	break large functions into chunked subfunctions
SDE 22	2020/07/10	replace wrapper functions with macros
SDE 23	2020/07/10	replace dimension array access with simple index
SDE 24	2020/07/10	optimize __lookup function

[chrispcampbell]

Merged to develop in 6ce8d49.