Allow custom from_word and from_none behaviors in derived FromMeta impls #1996

Job	Run time
Test Rust nightly on ubuntu-latest	22s
Test Rust nightly on macos-latest	28s
Test Rust nightly on windows-latest	53s
Verify code formatting	14s
Test Rust stable on ubuntu-latest	31s
Test Rust stable on macos-latest	31s
Test Rust stable on windows-latest	1m 48s
Test nightly with diagnostics feature	26s
Test Rust 1.56.0 on ubuntu-latest	4m 35s
Test Rust 1.56.0 on windows-latest	5m 27s
Lint with clippy	19s
	15m 34s

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