Make must_cast by-value and by-shared-ref functions const (but not by… #80

Job	Run time
Test with miri	1m 0s
Test on i686-unknown-linux-gnu with cross	1m 4s
Test with -Zsanitizer=address	15s
Test Rust 1.34.0 on ubuntu-latest	1m 43s
Test with -Zsanitizer=memory	14s
Test Rust stable on ubuntu-latest	10s
Test with -Zsanitizer=leak	11s
Test Rust beta on ubuntu-latest	14s
Test Rust nightly on ubuntu-latest	23s
Test Rust stable on macos-latest	13s
Test Rust stable on windows-latest	25s
Test Rust stable-x86_64-gnu on windows-latest	33s
Test Rust stable-i686-msvc on windows-latest	38s
Test Rust stable-i686-gnu on windows-latest	31s
	7m 34s

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