tpl: Allow the partial template func to return any type

This commit adds support for return values in partials.

This means that you can now do this and similar:

    {{ $v := add . 42 }}
    {{ return $v }}

Partials without a `return` statement will be rendered as before.

This works for both `partial` and `partialCached`.

Fixes #5783

compare/compare.go

@@ -20,6 +20,12 @@ type Eqer interface {
 	Eq(other interface{}) bool
 }
 
+// ProbablyEq is an equal check that may return false positives, but never
+// a false negative.
+type ProbablyEqer interface {
+	ProbablyEq(other interface{}) bool
+}
+
 // Comparer can be used to compare two values.
 // This will be used when using the le, ge etc. operators in the templates.
 // Compare returns -1 if the given version is less than, 0 if equal and 1 if greater than