- _propagateCancel is used only in _BaseCtx constructor -> inline it
  there. Being run in the constructor makes it clear that this code
  works on new _BaseCtx object with empty set of children.

- since _cancelFrom interacts with the code moved from _propagateCancel,
  also move it to be close to cancel propagation setup.

No functional changes, just plain code movement.

/trusted-by @jerome
/reviewed-on nexedi/pygolang!16