py: Make UNARY_OP_NOT a first-class op, to agree with Py not semantics.
Fixes #1684 and makes "not" match Python semantics. The code is also simplified (the separate MP_BC_NOT opcode is removed) and the patch saves 68 bytes for bare-arm/ and 52 bytes for minimal/. Previously "not x" was implemented as !mp_unary_op(x, MP_UNARY_OP_BOOL), so any given object only needs to implement MP_UNARY_OP_BOOL (and the VM had a special opcode to do the ! bit). With this patch "not x" is implemented as mp_unary_op(x, MP_UNARY_OP_NOT), but this operation is caught at the start of mp_unary_op and dispatched as !mp_obj_is_true(x). mp_obj_is_true has special logic to test for truthness, and is the correct way to handle the not operation.