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.

unix/mpconfigport.h

@@ -101,6 +101,7 @@
 #endif
 #define MICROPY_PY_CMATH            (1)
 #define MICROPY_PY_IO_FILEIO        (1)
+#define MICROPY_PY_IO_RESOURCE_STREAM (1)
 #define MICROPY_PY_GC_COLLECT_RETVAL (1)
 #define MICROPY_MODULE_FROZEN_STR   (1)