Combine _afterguard_ and _guard_, and update guard insertion of ECC

Conformity checks only have an afterguard, we want to combine this afterguard with guard so that we have a single primitive function, instead of having almost the same thing twice.

Additionally, similarly to type patterns, we want this guard to occur both before and after the conformity checked code so that we can keep the context in the future if we have better error messages.

Something like:

p1 = conformity check 1
...
pv = conformity check v

x1', .., xn' = guard (x, .., xn, p1, .., pv, "context");
res1, .., resm = _prf_ (x1', .., xn');
res1', .., resm' = guard (res1, .., resm, p1, .., pv, "context");