codegen+bootstrap+runtime: Use switch-statements in enum's debug_description + Remove Variant::visit

[Hendiadyoin1]

codegen: Use a switch statement in enum's debug_description

This is the last occurrence of Variant::visit, which may be putting some
strain on compilers template engines

runtime+bootstrap: Update stage0 and remove Variant::visit

This is as of the last commit unused and we have a better way of
implementing this through our compiler without relying on the
c++-compiler's template engine.

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I did not update the stage0 README. because the commit hashes may change during merging

[ilyapopov]

Performance measurement:

hyperfine "cd jakt/build/ && clang++ -std=c++20 -I ../runtime/ -Wno-parentheses-equality -Wno-trigraphs -Wno-user-defined-literals jakt.cpp" "cd jakt-pr1038/build/ && clang++ -std=c++20 -I ../runtime/ -Wno-parentheses-equality -Wno-trigraphs -Wno-user-defined-literals jakt.cpp"
Benchmark 1: cd jakt/build/ && clang++ -std=c++20 -I ../runtime/ -Wno-parentheses-equality -Wno-trigraphs -Wno-user-defined-literals jakt.cpp
  Time (mean ± σ):     14.773 s ±  0.077 s    [User: 14.204 s, System: 0.555 s]
  Range (min … max):   14.683 s … 14.953 s    10 runs
 
Benchmark 2: cd jakt-pr1038/build/ && clang++ -std=c++20 -I ../runtime/ -Wno-parentheses-equality -Wno-trigraphs -Wno-user-defined-literals jakt.cpp
  Time (mean ± σ):     13.734 s ±  0.068 s    [User: 13.232 s, System: 0.486 s]
  Range (min … max):   13.615 s … 13.860 s    10 runs
 
Summary
  'cd jakt-pr1038/build/ && clang++ -std=c++20 -I ../runtime/ -Wno-parentheses-equality -Wno-trigraphs -Wno-user-defined-literals jakt.cpp' ran
    1.08 ± 0.01 times faster than 'cd jakt/build/ && clang++ -std=c++20 -I ../runtime/ -Wno-parentheses-equality -Wno-trigraphs -Wno-user-defined-literals jakt.cpp'

So, this PR cuts about 1s (8%) from build time!

[alimpfard]

lgtm, just one nitpick.