py: Allow to properly disable builtin "set" object.

This patch makes MICROPY_PY_BUILTINS_SET compile-time option fully
disable the builtin set object (when set to 0).  This includes removing
set constructor/comprehension from the grammar, the compiler and the
emitters.  Now, enabling set costs 8168 bytes on unix x64, and 3576
bytes on stmhal.
py: Allow to properly disable builtin slice operation.

This patch makes the MICROPY_PY_BUILTINS_SLICE compile-time option
fully disable the builtin slice operation (when set to 0).  This
includes removing the slice sytanx from the grammar.  Now, enabling
slice costs 4228 bytes on unix x64, and 1816 bytes on stmhal.