Move ResourceMetadata into its own module, and give it a proper interface

[jimblandy]

Populating and removing ResourceMetadata elements should affect all three parallel arrays simultaneously. This is easiest to ensure (and show to readers) by making the members private and using a method for insertion. Accesses look cleaner with methods, too.

Checklist

• Run cargo clippy.
• Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

Connections
Link to the issues addressed by this PR, or dependent PRs in other repositories

Description
Describe what problem this is solving, and how it's solved.

Testing
Explain how this change is tested.

[codecov-commenter]

Codecov Report

Merging #3213 (f98c2dc) into master (04d12ba) will decrease coverage by 0.07%.
The diff coverage is 63.59%.

@@            Coverage Diff             @@
##           master    #3213      +/-   ##
==========================================
- Coverage   64.70%   64.63%   -0.08%     
==========================================
  Files          81       82       +1     
  Lines       38819    39495     +676     
==========================================
+ Hits        25118    25527     +409     
- Misses      13701    13968     +267     

Impacted Files	Coverage Δ
player/src/lib.rs	53.31% <ø> (ø)
wgpu-core/src/id.rs	81.25% <0.00%> (-2.63%)	⬇️
wgpu-core/src/lib.rs	96.55% <ø> (ø)
wgpu-core/src/present.rs	0.00% <0.00%> (ø)
wgpu-core/src/resource.rs	25.13% <ø> (ø)
wgpu-hal/src/dx11/device.rs	0.00% <0.00%> (ø)
wgpu-hal/src/empty.rs	0.00% <ø> (ø)
wgpu-hal/src/lib.rs	26.21% <ø> (ø)
wgpu/src/backend/direct.rs	54.80% <0.00%> (-0.30%)	⬇️
wgpu/src/lib.rs	50.33% <0.00%> (-2.49%)	⬇️
... and 45 more

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[cwfitzgerald]

Dope