Use onRangeError in StringMap.opIndex

[nordlow]

For the sake of having StringMap.opIndex behave like AA.opIndex and be nothrow @nogc.

[codecov-commenter]

Codecov Report

Merging #386 (99c812f) into master (b21ff32) will decrease coverage by 0.21%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #386      +/-   ##
==========================================
- Coverage   93.04%   92.83%   -0.22%     
==========================================
  Files          65       65              
  Lines       15891    15927      +36     
==========================================
  Hits        14786    14786              
- Misses       1105     1141      +36     

Impacted Files	Coverage Δ
source/mir/string_map.d	95.15% <0.00%> (-0.25%)	:arrow_down:
source/mir/interpolate/constant.d	71.42% <0.00%> (-15.53%)	:arrow_down:
source/mir/interpolate/generic.d	71.42% <0.00%> (-11.91%)	:arrow_down:
source/mir/rc/slim_ptr.d	69.01% <0.00%> (-7.55%)	:arrow_down:
source/mir/rc/ptr.d	76.92% <0.00%> (-6.42%)	:arrow_down:
source/mir/interpolate/linear.d	90.80% <0.00%> (-5.54%)	:arrow_down:
source/mir/interpolate/spline.d	91.70% <0.00%> (-1.14%)	:arrow_down:

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