expose expect function as well

Summary:
Currently you can use almost all functions via the exported `expects` struct. Though `expect` is not available. This would be nice to have as it makes writing expectations for custom checks a bit nicer when you're already using expects.

We can name is `expects.true` as well, or anything else that you'd recommend. For now I've named it `expects.expect`.

Reviewed By: IanChilds

Differential Revision: D68328897

fbshipit-source-id: ca01f1edd3107d03c77bc60619dcd181b79e70ec

prelude/utils/expect.bzl

@@ -126,6 +126,7 @@ def expect_struct(name: str, val: struct):
 
 # You'll likely want to import this struct for convenience, instead of each method separately
 expects = struct(
+    expect = expect,
     type = expect_type,
     string = expect_string,
     string_starts_with = expect_string_starts_with,