Start on the C api

[jneem]

This adds C bindings to the public API. I think the most important basics are covered (inspecting values, deep/shallow eval, printing errors).

I've also added some very basic tests, included in nix flake check. However, I haven't updated the release workflows to actually distribute anything. I'm not quite sure what (if anything) makes sense to distribute, actually. Maybe libnickel_lang.a and nickel_lang.h?

[github-actions]

Bencher Report

Branch	c-api
Testbed	ubuntu-latest

Click to view all benchmark results

Benchmark	Latency	microseconds (µs)
diagnostics-benches/inputs/goto-perf.ncl	📈 view plot 🚷 view threshold	10,051.00 µs
diagnostics-benches/inputs/large-record-tree.ncl	📈 view plot 🚷 view threshold	182,330.00 µs
diagnostics-benches/inputs/redis-replication-controller.ncl	📈 view plot 🚷 view threshold	297.84 µs
diagnostics-benches/inputs/small-record-tree.ncl	📈 view plot 🚷 view threshold	425.40 µs
fibonacci 10	📈 view plot 🚷 view threshold	254.60 µs
foldl arrays 50	📈 view plot 🚷 view threshold	445.83 µs
foldl arrays 500	📈 view plot 🚷 view threshold	3,688.20 µs
foldr strings 50	📈 view plot 🚷 view threshold	4,374.90 µs
foldr strings 500	📈 view plot 🚷 view threshold	42,813.00 µs
generate normal 250	📈 view plot 🚷 view threshold	39,426.00 µs
generate normal 50	📈 view plot 🚷 view threshold	1,538.50 µs
generate normal unchecked 1000	📈 view plot 🚷 view threshold	1,981.40 µs
generate normal unchecked 200	📈 view plot 🚷 view threshold	419.66 µs
init-diagnostics-benches/inputs/goto-perf.ncl	📈 view plot 🚷 view threshold	50,990.00 µs
init-diagnostics-benches/inputs/large-record-tree.ncl	📈 view plot 🚷 view threshold	200,810.00 µs
init-diagnostics-benches/inputs/redis-replication-controller.ncl	📈 view plot 🚷 view threshold	45,873.00 µs
init-diagnostics-benches/inputs/small-record-tree.ncl	📈 view plot 🚷 view threshold	45,748.00 µs
pidigits 100	📈 view plot 🚷 view threshold	2,089.60 µs
pipe normal 20	📈 view plot 🚷 view threshold	663.98 µs
pipe normal 200	📈 view plot 🚷 view threshold	5,601.30 µs
product 30	📈 view plot 🚷 view threshold	247.17 µs
requests-benches/inputs/goto-perf.ncl-000	📈 view plot 🚷 view threshold	2,681.00 µs
requests-benches/inputs/large-record-tree.ncl-000	📈 view plot 🚷 view threshold	553,780.00 µs
requests-benches/inputs/large-record-tree.ncl-001	📈 view plot 🚷 view threshold	86.64 µs
scalar 10	📈 view plot 🚷 view threshold	554.46 µs
sum 30	📈 view plot 🚷 view threshold	247.61 µs

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