Support build target `wasm32-unknown-emscripten` (#179) #215

Job	Run time
Build on MSRV (1.48) (ubuntu-latest, x86_64-unknown-linux-gnu, 1.48.0)	1m 48s
Tests (macos-latest, x86_64-apple-darwin, stable)	1m 11s
Build on MSRV (1.48) (windows-latest, i686-pc-windows-msvc, 1.48.0)	2m 9s
Tests (ubuntu-latest, x86_64-unknown-linux-gnu, 1.51.0)	1m 50s
Tests (ubuntu-latest, i686-unknown-linux-gnu, 1.51.0)	2m 20s
WASI (wasm32-wasi)	28s
Tests (windows-latest, i686-pc-windows-msvc, 1.51.0)	2m 32s
Tests (windows-latest, x86_64-pc-windows-msvc, 1.51.0)	2m 32s
Tests (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	32s
WASM Check (wasm32-unknown-unknown, ubuntu-latest)	27s
Tests (ubuntu-latest, x86_64-unknown-linux-gnu, beta)	32s
WASM Check (wasm32-unknown-emscripten, ubuntu-latest)	14s
WASM Check (wasm32-unknown-emscripten, windows-latest)	19s
WASM Check (wasm32-unknown-emscripten, macos-latest)	25s
Nightly Tests	30s
	17m 49s

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