Don't parse runtime package

Parsing the runtime package was previously done
to avoid discrepancies between frontend and
runtime definitions of runtime functions and
types. This does carry a cost of tight coupling,
and increases the startup overhead of the
compiler.

We now define all runtime functions in the
compiler, and no longer parse the runtime
package.
Merge pull request #129 from axw/libgo-no-runtime-parse

Don't parse runtime package