Remove all dict/vec/varray/darray/shape/tuple implicit conversions in assert path

[fredemmott]

Related: #48, #18

We're expecting to make darray an alias for dict, varray an alias for vec, and swap the backing store for shapes and tuples to dict/vec respectively in the next 2-8 weeks. Once that has happened in HHVM:

• this implicit conversion should be removed
• we should start asserting that a successful assert returns exactly the same value, just with a different type