Add "zero or" functions for `Sign`. #214

	name: CI
	on: [pull_request, push]
	jobs:
	rustfmt:
	name: Format
	runs-on: ubuntu-latest
	steps:
	- uses: actions/checkout@v2
	- uses: actions-rs/toolchain@v1
	with:
	profile: minimal
	toolchain: nightly
	override: true
	components: rustfmt
	- uses: actions-rs/cargo@v1
	with:
	command: fmt
	args: --all -- --check
	clippy:
	name: Lint
	runs-on: ubuntu-latest
	steps:
	- uses: actions/checkout@v2
	- uses: actions-rs/toolchain@v1
	with:
	profile: minimal
	toolchain: stable
	override: true
	components: clippy
	- run: rustup component add clippy
	- uses: actions-rs/cargo@v1
	with:
	command: clippy
	args: --all-features --all-targets -- -D clippy::all
	test:
	name: Test
	needs: [clippy, rustfmt]
	runs-on: ${{ matrix.os }}
	strategy:
	fail-fast: false
	matrix:
	os: [macOS-latest, ubuntu-latest, windows-latest]
	toolchain:
	- 1.70.0 # Minimum.
	- stable
	- beta
	steps:
	- uses: actions/checkout@v2
	- uses: actions-rs/toolchain@v1
	with:
	profile: minimal
	toolchain: ${{ matrix.toolchain }}
	override: true
	- uses: actions-rs/cargo@v1
	with:
	command: test
	args: --no-default-features --verbose
	- uses: actions-rs/cargo@v1
	with:
	command: test
	args: --all-features --verbose

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