Start support for array sum optimisation

[richfitz]

This PR adds support for an alternative way of writing out partial sums (i.e., creating a lower dimensional tensor from a higher dimensional one by summing over some dimensions).

Currently supports expressions that can be factorised as addition of a series of terms which may or may not include partial sums.

• Implement support for subtraction
• Implement R code generation (?)
• Test for unary minus
• Test for impossible sums (just add parens, incomplete sums)
• Are our variable dependencies ok?

Fixes #235

[codecov]

Codecov Report

Merging #237 (9373934) into master (b209499) will decrease coverage by 0.23%.
The diff coverage is 93.88%.

@@             Coverage Diff             @@
##            master     #237      +/-   ##
===========================================
- Coverage   100.00%   99.76%   -0.24%     
===========================================
  Files           39       39              
  Lines         4430     4594     +164     
===========================================
+ Hits          4430     4583     +153     
- Misses           0       11      +11     

Impacted Files	Coverage Δ
R/generate_c_equation.R	99.11% <90.32%> (-0.89%)	⬇️
R/ir_parse_arrays.R	98.65% <92.92%> (-1.35%)	⬇️
R/generate_c_sexp.R	100.00% <100.00%> (ø)
R/ir_parse.R	100.00% <100.00%> (ø)
R/ir_serialise.R	100.00% <100.00%> (ø)
R/odin_options.R	100.00% <100.00%> (ø)

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