Correct and test compiled comparison in package

[richfitz]

In trying to wire this up with sircovid it turned out I'd failed to test (or implement correctly) the package interface to this.

This has become a bit of an omnibus PR with a number of bugs and unpleasantnesses fixed:

• generated code no longer includes whitespace (in C++), with test
• moved away from glue for the odin substitution because it's escape behaviour mangled code ()) became ) because it was an exactly-doubled closing marker)
• generating the data struct constructor was broken if it contained more than one element
• odin_dust() did not work with compare function when the source was in a subdirectory

Fixes #55

[codecov[bot]]

Codecov Report

Merging #56 (7366fc8) into master (bf49eb8) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #56   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          656       697   +41     
=========================================
+ Hits           656       697   +41     

Impacted Files	Coverage Δ
R/generate_dust.R	100.00% <100.00%> (ø)
R/odin_dust.R	100.00% <100.00%> (ø)
R/package.R	100.00% <100.00%> (ø)
R/utils.R	100.00% <100.00%> (ø)

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