compiler: return error messages with source location

Replace most errors returned by the compiler (using errors.New) with an
error type that includes the source location.

compiler/errors.go

@@ -0,0 +1,14 @@
+package compiler
+
+import (
+	"go/token"
+	"go/types"
+)
+
+func (c *Compiler) makeError(pos token.Pos, msg string) types.Error {
+	return types.Error{
+		Fset: c.ir.Program.Fset,
+		Pos:  pos,
+		Msg:  msg,
+	}
+}