py/modio: Implement uio.resource_stream(package, resource_path).
The with semantics of this function is close to pkg_resources.resource_stream() function from setuptools, which is the canonical way to access non-source files belonging to a package (resources), regardless of what medium the package uses (e.g. individual source files vs zip archive). In the case of MicroPython, this function allows to access resources which are frozen into the executable, besides accessing resources in the file system. This is initial stage of the implementation, which actually doesn't implement "package" part of the semantics, just accesses frozen resources from "root", or filesystem resource - from current dir.