dependency is not a wildcard.

Related: #110

Also fixes treatment for self-dependencies in wildcards (closes #111).

move host code generation to ExecutuableCTarget, add test to ensure we generate valid code with global temporaries present for C

Add test that casting to an invalid type raises LoopyError.

Disallow casting a non-numeric subtype to a numeric type.

This allows invocation of pytest from an arbitrary directory to
succeed in running the test.

Fixes #99.