feat!: Lower string, print, and panic

[mark-koch]

Closes #79.

Adds lowering logic for:

• String type and ConstString (c-style)
• Error type and ConstError
• Print op (lowers to libc printf by default)
• Panic op (lowers to libc abort by default)

BREAKING CHANGE: CodegenExtension::emitter has a new lifetime bound on the self argument. PreludeCodegen trait now requires Clone.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.24590% with 18 lines in your changes missing coverage. Please review.

Project coverage is 82.97%. Comparing base (ef6e337) to head (9f8b0c7).

Files	Patch %	Lines
src/custom/prelude.rs	86.53%	4 Missing and 10 partials ⚠️
src/emit/libc.rs	77.77%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #78      +/-   ##
==========================================
+ Coverage   82.54%   82.97%   +0.42%     
==========================================
  Files          17       18       +1     
  Lines        2280     2390     +110     
  Branches     2280     2390     +110     
==========================================
+ Hits         1882     1983     +101     
+ Misses        258      253       -5     
- Partials      140      154      +14     

Flag	Coverage Δ
rust	82.97% <85.24%> (+0.42%)	⬆️

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[qartik]

LGTM