Add PlatformEncoding and Utf8PlatformEncoding #114

Job	Run time
Run examples using Rust stable (ubuntu-latest)	7s
Verify code formatting (windows-latest)	40s
Lint with clippy (windows-latest, --all-features)	1m 26s
Lint with clippy (windows-latest, --no-default-features)	1m 12s
Verify code formatting (ubuntu-latest)	9s
Lint with clippy (ubuntu-latest, --all-features)	6s
Test Rust stable (windows-latest, x86_64-pc-windows-msvc --all-features)	1m 15s
Test Rust stable (windows-latest, x86_64-pc-windows-msvc --no-default-features)	51s
Lint with clippy (ubuntu-latest, --no-default-features)	6s
Test Rust stable (macos-latest, --all-features)	24s
Test Rust stable (macos-latest, --no-default-features)	8s
Test Rust stable (ubuntu-latest, --all-features)	31s
Test Rust stable (ubuntu-latest, --no-default-features)	8s
Test Rust stable (ubuntu-latest, wasm32-unknown-unknown --all-features)	6s
Test Rust stable (ubuntu-latest, wasm32-unknown-unknown --no-default-features)	9s
Test Rust 1.58.1 (ubuntu-latest, --all-features)	47s
Test Rust 1.58.1 (ubuntu-latest, --no-default-features)	18s
	8m 23s

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