refactor(tor): rewrite using measurex (#652)

This diff rewrites the tor experiment to use measurex "easy" API.

To this end, we need to introduce an "easy" measurex API, which basically
performs easy measurements returning two pieces of data:

1. the resulting measurement, which is already using the OONI
archival data format and is always non-nil

2. a failure (i.e., the pointer to an error string), which
is nil on success and points to a string on failure

With this change, we should now be able to completely dispose of
the original netx API, which was only used by tor.

Reference issue: https://github.com/ooni/probe/issues/1688.

internal/runtimex/runtimex.go

@@ -22,3 +22,8 @@ func PanicIfFalse(assertion bool, message string) {
 func PanicIfTrue(assertion bool, message string) {
 	PanicIfFalse(!assertion, message)
 }
+
+// PanicIfNil calls panic if the given interface is nil.
+func PanicIfNil(v interface{}, message string) {
+	PanicIfTrue(v == nil, message)
+}