Add `fix` and `unfix` functions for parameter fixing and unfixing #241

	name: IntegrationTest

	on:
	push:
	branches:
	- master
	pull_request:

	jobs:
	test:
	name: ${{ matrix.package.repo }}
	runs-on: ubuntu-latest
	strategy:
	fail-fast: false
	matrix:
	package:
	- {user: TuringLang, repo: DynamicPPL.jl}
	steps:
	- uses: actions/checkout@v2
	- uses: julia-actions/setup-julia@v1
	with:
	version: 1
	arch: x64
	- uses: julia-actions/julia-buildpkg@latest
	- name: Clone Downstream
	uses: actions/checkout@v2
	with:
	repository: ${{ matrix.package.user }}/${{ matrix.package.repo }}
	path: downstream
	- name: Load this and run the downstream tests
	shell: julia --color=yes --project=downstream {0}
	run: \|
	using Pkg
	try
	# force it to use this PR's version of the package
	Pkg.develop(PackageSpec(path=".")) # resolver may fail with main deps
	Pkg.update()
	Pkg.test() # resolver may fail with test time deps
	catch err
	err isa Pkg.Resolve.ResolverError \|\| rethrow()
	# If we can't resolve that means this is incompatible by SemVer and this is fine
	# It means we marked this as a breaking change, so we don't need to worry about
	# Mistakenly introducing a breaking change, as we have intentionally made one
	@info "Not compatible with this release. No problem." exception=err
	exit(0) # Exit immediately, as a success
	end

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