This adds type checking to CallExpr (excluding builtin type checking, as that is a PR in its own right) as well as handling any required constant type conversion.
This also changes constant strings and runes to be represented as `constant.Value`. Runes change `rval` type at CFG typing time to avoid having to type at AST time. There are also changes to importSpecs and `stdlib` to account for the string change. With this all `untyped` types should now be `constant.Value`s, although errors are still not returned if this is not the case to be sure we do not break things.
This also fixed a bug in `itype.methods` that would panic if the type was recursive.
* fix: make interpreter methods discoverable by runtime
When generating an interface wrapper, lookup existing wrappers by method
to get the one with the biggest set of methods implemented by interpreter.
A string method is also added to wrappers, in order to provide a string
representation of the interpreter value rather than the wrapper itself
(at least for %s and %v verbs).
This allows the runtime to pickup an interpreter method automatically
even if the conversion to the interface is not specified in the script. As
in Go spec, it is enough for the type to implement the required methods.
A current limitation is that only single wrappers can be instantiated,
not allowing to compose interfaces.
This limitation can be removed when the Go reflect issue
https://github.com/golang/go/issues/15924 is fixed.
Fixes#435.
* test: add a simpler test
