Function centric codegen

[rachel-bousfield]

Makes codegen function-centric so that codegen is module-agnostic.
This makes future optimizations possible and enables other refactors to codgen.

This PR also

• Reduces the number of backward jumps from 279 to 11 for a 0.5% performance gain
• Reduces the number of AVM instructions in ArbOS by 12.88%
• Multi-threads codegen
• Creates .dot graph files for callgraph viewing
• Globals can now have attributes & debug info
• Refactors unused function procedure to not generate code for stdlib funcs
• Refactors label_gen to be less stateful
• Removes lifetime parameters from codegen
• Removes all code relocation & simplifies linking
• Adds the string datatype
• Reduces special-casing around closures
• Improves import error messages and logic

[rachel-bousfield]

The nightly tests are failing due to a bug in the rust compiler.

[codecov-commenter]

Codecov Report

Merging #594 (8b465a3) into next (8792056) will increase coverage by 0.25%.
The diff coverage is 86.91%.

@@            Coverage Diff             @@
##             next     #594      +/-   ##
==========================================
+ Coverage   77.40%   77.66%   +0.25%     
==========================================
  Files          88       88              
  Lines       29012    28694     -318     
==========================================
- Hits        22458    22286     -172     
+ Misses       6554     6408     -146     

[RGates94]

LGTM