Fix up strict-assert usage

[cwfitzgerald]

Checklist

• Run cargo clippy.
• Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.

Connections

Follow up to #3163

Description

Fixes a couple of small issues with strict-asserts.

• Enabling wgpu/strict-asserts would unconditionally pull in wgpu-core.
• wgpu-core still had it's assertions.rs file, despite the asserts moving to wgpu-types
• strict_unwrap_unchecked was missing unsafe on the caller
• Added more documentation to the assertion module.
• Strict asserts shouldn't be default on in the wgpu crate.

Testing
None

[codecov-commenter]

Codecov Report

Merging #3320 (df4a1e1) into master (4c5fef4) will decrease coverage by 0.07%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master    #3320      +/-   ##
==========================================
- Coverage   64.18%   64.11%   -0.08%     
==========================================
  Files          86       86              
  Lines       42553    42557       +4     
==========================================
- Hits        27313    27285      -28     
- Misses      15240    15272      +32     

Impacted Files	Coverage Δ
wgpu-types/src/assertions.rs	37.50% <0.00%> (-12.50%)	⬇️
wgpu-hal/src/dx12/suballocation.rs	85.92% <100.00%> (+0.42%)	⬆️
wgpu-core/src/track/buffer.rs	84.86% <0.00%> (-4.22%)	⬇️
wgpu-core/src/track/metadata.rs	89.47% <0.00%> (-1.51%)	⬇️
wgpu-core/src/track/texture.rs	59.46% <0.00%> (-0.50%)	⬇️
wgpu/src/context.rs	52.73% <0.00%> (-0.11%)	⬇️

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