Misc improvements to the @resumable macro

[Krastanov]

• more descriptive names generated symbols
• enable docstrings for @resumable functions
• do not delete line number nodes in @resumable functions

Fixes #39

Fixes #45

Initial steps needed to address #46

[codecov-commenter]

Codecov Report

Merging #71 (84c2c51) into master (727ba4a) will decrease coverage by 6.27%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
- Coverage   93.49%   87.22%   -6.27%     
==========================================
  Files           5        5              
  Lines         292      321      +29     
==========================================
+ Hits          273      280       +7     
- Misses         19       41      +22     

Files Changed	Coverage Δ
src/macro.jl	87.23% <100.00%> (ø)
src/transforms.jl	79.72% <100.00%> (-14.14%)	⬇️

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[Krastanov]

The coverage calculations seem to be wrong... But the previous coverage calculations were probably wrong too. The changes happened because of the various modifications I have made in order to preserve Line Number Nodes, which are necessary for correct coverage tracking. Further improvements are necessary for this to properly track coverage, but this should be better than the status quo.

[Krastanov]

One can verify that the coverage reporting is nonsense here: https://app.codecov.io/gh/JuliaDynamics/ResumableFunctions.jl/pull/71/blob/src/transforms.jl -- e.g. documentation strings and lines containing only end are marked as not covered...