* Implemented Native function as IExprs so we can refer to them
like any other function. Checkout `NativeFunction`.
* `require` has been implemented as a native functions which
accepts one or more namespace descriptions and load them.
A namespace description is either a symbol with the ns name
or an `IColl` in the following form `(some.ns.name alias).
* A hashmap of strings to native functions has been added to the Runtime
which is used to resolve builtin functions.
Sameer Rahmani