remove global metadata cache, refactor custom_metadata API

[jrevels]

essentially implements the approach described here #90 (comment)

I haven't run tests fully locally yet so may still have some lingering bugs. They pass 😎

I also haven't been super performance-conscious here so we should be careful to make sure regressions aren't being introduced.

Since we're doing a major bump for this anyway, I wonder whether it's a good time to fully delete some of the lingering deprecations here.

closes #90, #211

[codecov]

Codecov Report

Merging #238 (7f74cdd) into main (7d59427) will increase coverage by 2.02%.
The diff coverage is 92.30%.

@@            Coverage Diff             @@
##             main     #238      +/-   ##
==========================================
+ Coverage   83.79%   85.81%   +2.02%     
==========================================
  Files          26       26              
  Lines        3259     3265       +6     
==========================================
+ Hits         2731     2802      +71     
+ Misses        528      463      -65     

Impacted Files	Coverage Δ
src/Arrow.jl	56.52% <ø> (-1.82%)	⬇️
src/ArrowTypes/src/ArrowTypes.jl	77.92% <ø> (+3.07%)	⬆️
src/arraytypes/bool.jl	88.70% <ø> (+1.61%)	⬆️
src/arraytypes/fixedsizelist.jl	84.53% <ø> (+4.12%)	⬆️
src/arraytypes/list.jl	92.24% <ø> (+3.87%)	⬆️
src/arraytypes/map.jl	100.00% <ø> (ø)
src/arraytypes/primitive.jl	84.31% <ø> (ø)
src/arraytypes/struct.jl	100.00% <ø> (+1.53%)	⬆️
src/arraytypes/unions.jl	94.32% <ø> (+2.12%)	⬆️
src/write.jl	94.57% <80.00%> (-0.06%)	⬇️
... and 22 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7d59427...7f74cdd. Read the comment docs.

[jrevels]

Flipping out of draft as tests pass and docs are updated (unless I missed anything).

Before merging, should we decide on:

Since we're doing a major bump for this anyway, I wonder whether it's a good time to fully delete some of the lingering deprecations here.

We also could do this in a separate PR before tagging

[ericphanson]

exciting! Didn't know about Base.ImmutableDict, cool.

[quinnj]

LGTM; thanks for taking this on! I left one suggestion to simplify toiddict.

[jrevels]

From Slack convo:

I'll merge this PR once CI passes and can open a follow-up later to delete the deprecated regions before we actually tag the v2.0 release.

[jrevels]

Hmm. Looks like Julia 1.3 tests now fail because the foldl implementation of toidict relies on Base.ImmutableDict(::Pair, ::Pair) working, which I guess isn't defined on Julia 1.3

and the Julia 1.6 tests fail because foldl doesn't apply the given op on unary inputs (which is expected):

julia> foldl(Base.ImmutableDict, ("a" => "b",))
"a" => "b"

I think I'll just revert back to the original more verbose implementation, which also has that slight performance benefit anyway