pkg/runtime: fix convertE2I

Set the correct rtyp.
More work on interfaces

Runtime types are now generated as well as
they are in the master branch.
Moar interfaces

Now only interfaces/error.go is failing, and
recursive pointers are broken (regression?)
Fix convertE2I: pass underlying type to runtime
Only add exported methods to runtime type
Don't branch in struct comparisons

If we change the CFG that go.tools/ssa emits, then it
becomes more difficult to translate its instructions;
for example, Phi's incoming edges may have to change.
We will avoid modifying the CFG.

For struct comparisons, we're temporarily emitting
code to unconditionally compare all fields and then
combining the results with bitwise-AND. We'll later
use the struct's runtime struct algorithm, which may
be inlined by LLVM (across modules, if done right).

Also: fixed runtime type generation for named types;
methods weren't being populated.