py: Add MICROPY_ENABLE_DOC_STRING, disabled by default.

Also add a few STATIC's to some compile functions that should have them.

Addresses issue #521.
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 4c3e070..b669128 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -89,6 +89,11 @@
 #define MICROPY_ENABLE_SOURCE_LINE (0)
 #endif
 
+// Whether to include doc strings (increases RAM usage)
+#ifndef MICROPY_ENABLE_DOC_STRING
+#define MICROPY_ENABLE_DOC_STRING (0)
+#endif
+
 // Float and complex implementation
 #define MICROPY_FLOAT_IMPL_NONE (0)
 #define MICROPY_FLOAT_IMPL_FLOAT (1)