c-api: Remove allocations from `wasmtime_val_t` #22929

Job	Run time
Rustfmt	30s
Clang format	14s
Determine CI jobs to run	21s
Check	13m 4s
Check Windows ARM64	0s
Cargo vet	0s
Cargo deny	0s
Doc build	0s
Test C-API ubuntu-latest	7m 20s
Test Linux x86_64	14m 48s
Test C-API macos-latest	15m 43s
Run benchmarks	0s
Build wasi-preview1-component-adapter	0s
Test C-API macos-14	4m 24s
Test C-API windows-latest	10m 22s
Build wasmtime-target-wasm32	0s
Nightly tests	0s
Release build for ${{ matrix.build }}	0s
Meta deterministic check	0s
Miri	0s
verify-publish	0s
Cargo vet failed on a Pull Request	0s
Record the result of testing and building steps	1s
push-tag	0s
	1h 6m 47s

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