Clang.jl generated bindings and internal refactor

[metab0t]

A first step of #295

1. Use Clang.jl to generate C bindings automatically
2. Use WeakKeyDict to record prepared statements of a DB
3. Upgrade Julia version requirement to 1.6

All tests pass locally.

[metab0t]

@quinnj
JuliaFormatter.jl causes extra diff but the runtests.jl is almost unaffected (no public API change).
So maybe we can merge it as long as CI is green?

[metab0t]

The formatter causes a lot of noisy diffs, I can open another PR to format existing code as Step 0 then merge this PR if we agree on the formatter style configuration.

[codecov]

Codecov Report

Merging #296 (14e47d9) into master (a4e4da3) will decrease coverage by 30.28%.
The diff coverage is 93.10%.

@@             Coverage Diff             @@
##           master     #296       +/-   ##
===========================================
- Coverage   85.88%   55.59%   -30.29%     
===========================================
  Files           5        5               
  Lines         758     1108      +350     
===========================================
- Hits          651      616       -35     
- Misses        107      492      +385     

Impacted Files	Coverage Δ
src/capi.jl	15.69% <ø> (ø)
src/UDF.jl	86.36% <86.20%> (+0.20%)	⬆️
src/SQLite.jl	95.94% <93.69%> (+1.88%)	⬆️
src/tables.jl	99.34% <96.55%> (-0.01%)	⬇️
src/base.jl	100.00% <100.00%> (ø)

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[metab0t]

@quinnj
Now #297 has merged. The diff is nicer to review except for 3k+ lines added by automatically generated C API bindings 😂

[quinnj]

This is great; thanks @metab0t!