That one was missing in the module, even if it was available in the
vfs object. The change consist of adding the name and preparing the
call to the underlying vfs module, similar to what was already
implemented e.g. for remove.
Rename is useful by itself, or for instance for a safe file replace,
consisting of the sequence:
write to a temp file
delete the original file
rename the temp file to the original file's name
These symbols are still defined in terms of the system Exxx symbols, and
can be switched to internal numeric definitions at a later stage.
Note that extmod/modlwip still uses many system Exxx symbols.
