This used to operate via some fast-and-loose (ab)use of the Trifecta
API. A simpler way to do things is to use streaming-bytestring to load
a file line by line and parse directives until we encounter program text.
Due to the problems outlined in #245, the tests for return statements
were complicated and not testing useful properties. This patch adds a
new `CHECK-TREE` directive which lets you embed a Core expression
as a string, which is parsed and then compared against the result of
compiling the containing module.
