fix(codegen): only generate exposed functions

[Wodann]

This also refactors codegen tests to always include symbol generation in
snapshot tests.

fixes issue #133

[codecov]

Codecov Report

Merging #134 into master will increase coverage by 2.87%.
The diff coverage is 89.46%.

@@            Coverage Diff             @@
##           master     #134      +/-   ##
==========================================
+ Coverage   78.86%   81.73%   +2.87%     
==========================================
  Files         141      141              
  Lines        8754     9035     +281     
==========================================
+ Hits         6904     7385     +481     
+ Misses       1850     1650     -200     

Impacted Files	Coverage Δ
crates/mun/src/main.rs	0.00% <0.00%> (ø)
crates/mun_compiler/src/driver/config.rs	100.00% <ø> (ø)
crates/mun_compiler/src/lib.rs	0.00% <0.00%> (ø)
crates/mun_compiler_daemon/src/lib.rs	0.00% <0.00%> (ø)
crates/mun_syntax/src/syntax_error.rs	66.66% <50.00%> (-6.07%)	⬇️
crates/mun_compiler/src/driver/display_color.rs	72.72% <72.72%> (ø)
crates/mun_compiler/src/diagnostics.rs	92.92% <91.91%> (+66.69%)	⬆️
crates/mun_compiler/src/diagnostics_snippets.rs	91.97% <91.97%> (ø)
crates/mun_compiler/src/driver.rs	78.84% <92.30%> (+8.63%)	⬆️
crates/mun_compiler/src/annotate.rs	94.31% <94.31%> (ø)
... and 26 more

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