Allow colon-equal to override variable type

If I execute the following:

```
I := interp.New(interp.Options{})
I.Eval(`x := 1`)
I.Eval(`x := "foobar"`)
```

I expect the second declaration to override the first. `var x string` will override the previous type, and redeclaring a type, function, const, etc will override it, but the `:=` operator will not.

Currently, the result is: `reflect.Set: value of type string is not assignable to type int`

interp/gta.go

@@ -80,9 +80,7 @@ func (interp *Interpreter) gta(root *node, rpath, importPath string) ([]*node, e
 				if typ.isBinMethod {
 					typ = &itype{cat: valueT, rtype: typ.methodCallType(), isBinMethod: true, scope: sc}
 				}
-				if sc.sym[dest.ident] == nil || sc.sym[dest.ident].typ.incomplete {
-					sc.sym[dest.ident] = &symbol{kind: varSym, global: true, index: sc.add(typ), typ: typ, rval: val, node: n}
-				}
+				sc.sym[dest.ident] = &symbol{kind: varSym, global: true, index: sc.add(typ), typ: typ, rval: val, node: n}
 				if n.anc.kind == constDecl {
 					sc.sym[dest.ident].kind = constSym
 					if childPos(n) == len(n.anc.child)-1 {

interp/interp_eval_test.go

@@ -123,6 +123,7 @@ func TestEvalAssign(t *testing.T) {
 		{src: "h := 1; h >>= 8", res: "0"},
 		{src: "i := 1; j := &i; (*j) = 2", res: "2"},
 		{src: "i64 := testpkg.val; i64 == 11", res: "true"},
+		{pre: func() { eval(t, i, "k := 1") }, src: `k := "Hello world"`, res: "Hello world"}, // allow reassignment in subsequent evaluations
 	})
 }
 
@@ -139,8 +140,8 @@ func TestEvalBuiltin(t *testing.T) {
 		{src: `g := cap(a)`, res: "1"},
 		{src: `g := len("test")`, res: "4"},
 		{src: `g := len(map[string]string{"a": "b"})`, res: "1"},
-		{src: `a := len()`, err: "not enough arguments in call to len"},
-		{src: `a := len([]int, 0)`, err: "too many arguments for len"},
+		{src: `n := len()`, err: "not enough arguments in call to len"},
+		{src: `n := len([]int, 0)`, err: "too many arguments for len"},
 		{src: `g := cap("test")`, err: "1:37: invalid argument for cap"},
 		{src: `g := cap(map[string]string{"a": "b"})`, err: "1:37: invalid argument for cap"},
 		{src: `h := make(chan int, 1); close(h); len(h)`, res: "0"},