lang: unification: Improve type unification algorithm

The simple type unification algorithm suffered from some serious
performance and memory problems when used with certain code bases. This
adds some crucial optimizations that improve performance drastically.

lang/interfaces/unification.go

@@ -19,6 +19,8 @@ package interfaces
 
 import (
 	"fmt"
+
+	"github.com/purpleidea/mgmt/lang/types"
 )
 
 // Invariant represents a constraint that is described by the Expr's and Stmt's,
@@ -27,4 +29,11 @@ import (
 type Invariant interface {
 	// TODO: should we add any other methods to this type?
 	fmt.Stringer
+
+	// ExprList returns the list of valid expressions in this invariant.
+	ExprList() []Expr
+
+	// Matches returns whether an invariant matches the existing solution.
+	// If it is inconsistent, then it errors.
+	Matches(solved map[Expr]*types.Type) (bool, error)
 }