Improve compilation latency by reducing recursion limit

[tkf]

An alternative fix to #412.

Compilation benchmark using the example in #412:

FOLDL_RECURSION_LIMIT	w/ Julia 1.6.0-DEV.536	w/ Julia 1.5	w/ Julia 1.4
0	5 sec	8 sec	6 sec
1	6 sec	15 sec	8 sec
2	7 sec	21 sec	10 sec
3	8 sec	30 sec	12 sec
4	9 sec	34 sec	14 sec

FOLDL_RECURSION_LIMIT = Val(0) slows down ["teerf_filter", "noinit"] (and speedups ["filter_sum", "1000", "RandomFloats", "noinit", "base"]):

ID	time ratio	memory ratio
["filter_sum", "1000", "RandomFloats", "noinit", "base"]	0.31 (5%) ✅	1.00 (1%)
["teerf_filter", "noinit"]	226.80 (5%) ❌	832.57 (1%) ❌

--- https://github.com/JuliaFolds/Transducers-data/blob/benchmark-results/2020/08/10/070744/result.md

Interestingly, FOLDL_RECURSION_LIMIT = Val(1) improves the performance:

ID	time ratio	memory ratio
["filter_sum", "1000", "RandomFloats", "noinit", "base"]	0.26 (5%) ✅	1.00 (1%)

--- https://github.com/JuliaFolds/Transducers-data/blob/benchmark-results/2020/08/10/064257/result.md

Re: ["filter_sum", "1000", "RandomFloats", "noinit", "base"], see #403.

Commit Message

Improve compilation latency by reducing recursion limit (#412)

The tail-call pattern introduced in eb430cf
(#403) for linear indexing arrays seem to invoke a large compiler
latency (see #412 for an example). This patch tries to mitigate the
problem by reducing the FOLDL_RECURSION_LIMIT to 1. This still
allows two recursive calls. All the benchmarks do not show
noticeable degradation (presumably because recursion deeper than this
is not exercised in the benchmarks). Note that setting it to 0 (one
recursive call) shows 2x slowdown in ["teerf_filter", "noinit"]
benchmark.

[codecov]

Codecov Report

Merging #413 into master will increase coverage by 1.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #413      +/-   ##
==========================================
+ Coverage   89.52%   90.84%   +1.31%     
==========================================
  Files          25       25              
  Lines        1585     1594       +9     
==========================================
+ Hits         1419     1448      +29     
+ Misses        166      146      -20     

Flag	Coverage Δ
#unittests	90.84% <ø> (+1.31%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/processes.jl	93.03% <ø> (ø)
src/library.jl	93.15% <0.00%> (-0.28%)	⬇️
src/show.jl	89.51% <0.00%> (+0.07%)	⬆️
src/unordered.jl	96.61% <0.00%> (+0.31%)	⬆️
src/progress.jl	89.79% <0.00%> (+1.02%)	⬆️
src/core.jl	85.56% <0.00%> (+1.06%)	⬆️
src/groupby.jl	82.14% <0.00%> (+5.77%)	⬆️
src/reduce.jl	84.55% <0.00%> (+9.14%)	⬆️
src/basics.jl	92.10% <0.00%> (+10.52%)	⬆️

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