Add singleton support

[Wynand]

Currently singletons don't work with lower, and this adds support for that

Effectively the singletons are printed as their class with "()" as a suffix to show that they're an instance of the singleton instead of the singleton type

[codecov]

Codecov Report

Merging #331 (2258c48) into master (f93688a) will decrease coverage by 0.19%.
The diff coverage is 85.71%.

❗ Current head 2258c48 differs from pull request most recent head 00e171f. Consider uploading reports for the commit 00e171f to get more accurate results

@@            Coverage Diff             @@
##           master     #331      +/-   ##
==========================================
- Coverage   99.51%   99.31%   -0.20%     
==========================================
  Files           5        5              
  Lines         410      440      +30     
==========================================
+ Hits          408      437      +29     
- Misses          2        3       +1     

Impacted Files	Coverage Δ
src/Writer.jl	97.45% <85.71%> (-0.68%)	⬇️
src/Parser.jl	100.00% <0.00%> (ø)
src/specialized.jl	100.00% <0.00%> (ø)

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[Wynand]

This issue may be relevant: #203