Symbolic Compositional Resource Sharers

[olynch]

This PR is for the draft of symbolic, compositional resource sharers, and also includes a new namespacing mechanism via Tries.

[jpfairbanks]

I like the approach here to supporting namespaced variables, but I really think that people will get confused by the name Package not referring to a package of code. Can we use Namespace or Names instead of package?

[epatters]

Agreed on the name. I think it's too confusing.

[olynch]

I'm happy not calling it Package, but I don't like namespace. The point is that Package doesn't just include the names; it also includes an assignment of those names to values. A package has a namespace, which is the set of valid index paths. We could just call it a trie, which is what it is.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.36364% with 84 lines in your changes missing coverage. Please review.

Project coverage is 92.14%. Comparing base (bd3df8e) to head (41a9fc3).

Files	Patch %	Lines
src/stdlib/models/twosorted.jl	0.00%	27 Missing :warning:
src/syntax/gats/closures.jl	73.77%	16 Missing :warning:
src/syntax/gats/ast.jl	82.08%	12 Missing :warning:
src/util/Dtrys.jl	95.15%	11 Missing :warning:
src/nonstdlib/dynamics/ResourceSharers.jl	94.79%	9 Missing :warning:
src/syntax/gats/algorithms.jl	61.11%	7 Missing :warning:
src/syntax/gats/exprinterop.jl	88.23%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #136 +/- ## ========================================== - Coverage 93.85% 92.14% -1.71% ========================================== Files 38 42 +4 Lines 2229 2802 +573 ========================================== + Hits 2092 2582 +490 - Misses 137 220 +83 ```

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