Add try_as_raw_ptr/as_raw_ptr implementations for Symbol to convert to raw pointer #246

Job	Run time
cross-ios-build (nightly, aarch64-apple-ios)	13s
cross-ios-build (nightly, x86_64-apple-ios)	13s
cross-ios-build (stable, aarch64-apple-ios)	9s
cross-ios-build (stable, x86_64-apple-ios)	8s
bare-cross-build (nightly, x86_64-unknown-haiku)	25s
native-test (nightly, ubuntu-latest)	12s
bare-cross-build (nightly, x86_64-unknown-openbsd)	25s
windows-test (nightly, x86_64-pc-windows-gnu)	1m 22s
native-test (nightly, windows-latest)	25s
bare-cross-build (nightly, x86_64-unknown-redox)	26s
windows-test (nightly, i686-pc-windows-gnu)	1m 20s
native-test (nightly, macOS-latest)	12s
bare-cross-build (nightly, x86_64-fuchsia)	26s
windows-test (stable, x86_64-pc-windows-gnu)	2m 19s
native-test (stable, ubuntu-latest)	6s
bare-cross-build (nightly, wasm32-unknown-unknown)	23s
windows-test (stable, i686-pc-windows-gnu)	1m 59s
native-test (stable, windows-latest)	1m 23s
native-test (stable, macOS-latest)	6s
native-test (1.56.0, ubuntu-latest)	1m 55s
native-test (1.56.0, windows-latest)	2m 21s
native-test (1.56.0, macOS-latest)	1m 48s
	18m 16s

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