Replace Code.ensure_loaded? with Code.ensure_compiled! in Flint.Type to avoid race conditions

[venkatd]

Resolves #15

From the Elixir docs:

When invoked, ensure_compiled/1 and ensure_compiled!/1 halt the compilation of the caller until the module becomes available. Note that the distinction between ensure_compiled/1 and ensure_compiled!/1 is important: if you are using ensure_compiled!/1, you are indicating to the compiler that you can only continue if said module is available.

And then:

In most cases, ensure_loaded/1 is enough. ensure_compiled!/1 must be used in rare cases, usually involving macros that need to invoke a module for callback information. The use of ensure_compiled/1 is even less likely.

Code.ensure_loaded? does not guarantee that the module is available to call methods on. Because extending a type with Flint.Type requires calling functions on it, I believe Code.ensure_loaded? fixes the issue.

Our project is running on a fork for the time being.