Add array `contains` function

[jdidion]

Checklist

• [x] Pull request details were added to CHANGELOG.md
• [x] Valid examples WDL's were added or updated to the SPEC.md (see the guide on writing markdown tests)

[patmagee]

@jdidion do we need to make any comments here on how equality works and whether type coerced objects are equal?

Ie would the following be truthy?

File Foo = "foo"
Array[String] bar = ["foo"]

[jdidion]

@patmagee yes type coercion should apply here. String -> File is one of the officially supported coercions, but not File -> String.

Array[File] files = ["/foo/bar"]
contains(files, "/foo/bar")  # true

Array[String] strings = ["/foo/bar"]
File foobar = "/foo/bar"
contains(strings, foobar)  # false
contains(strings, "~{foobar}")  # true