py/objstr: More faithful and optimal implementation of empty format spec.
py/objstr: More faithful and optimal implementation of empty format spec.

The old code translated "{}" into "{!s}". It was thus implemented by
allocating auxiliary vstr, printing an object into it, then printing
aux vstr into the main vstr. This is unnecessary though, as an object
can be printed directly into the main vstr. This is especially true as
it's not allowed to specify arbitrary formatting features (field width,
etc.) when printing arbitrary objects, e.g. following is not allowed:

"{:.2}".format([1, 2])

(Unlike modulo operator, where "%.2s" % [1, 2] produces "[1").

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython