feat: add must helper for short form error checking

[hpidcock]

This adds a series of Must helpers that call a function on your behalf and
assert that the final error return value is nil. It will then return any remaining
return values to the caller of Must.

uuid := tc.Must(c, model.NewUUID)

Must has a series of forms available, MustX which takes X arguments to
pass to the function being called, returning a single argument. The MustX
series are aliases of the MustX_1 series. MustX_Y all take X arguments
and return Y values. The Must function is the most common case, a
function that takes no arguments, returns one value and an error.

[SimonRichardson]

This is better.