Make Xilem Core unconditionally `no_std` (#877) #2828

Job	Run time
cargo check (msrv) (wasm32)	22s
cargo clippy (windows-latest)	4m 11s
cargo check (aarch64-android)	29s
cargo clippy (macos-latest)	2m 21s
cargo check (msrv) (windows-latest)	1m 55s
cargo clippy (wasm32)	1m 51s
cargo clippy (ubuntu-latest)	3m 0s
cargo check (msrv) (macos-latest)	34s
cargo doc	43s
cargo check (msrv) (ubuntu-latest)	43s
formatting	40s
cargo miri (unsafe tree_arena)	34s
Prime LFS Cache	7s
cargo test (wasm32)	19s
typos	3s
cargo test (windows-latest)	4m 23s
cargo test (macos-latest)	1m 47s
cargo test (ubuntu-latest)	3m 17s
	27m 19s

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