interp: implement runtime.sliceCopy

This implements the copy() built-in function. It may not work in all
cases, but should work in most cases.

This commit gets the following 3 packages to compile, according to
tinygo-site/imports/main.go:

  * encoding/base32
  * encoding/base64
  * encoding/pem (was blocked by encoding/base64)

testdata/init.go

@@ -13,6 +13,11 @@ func main() {
 	println("v5:", len(v5), v5 == nil)
 	println("v6:", v6)
 	println("v7:", cap(v7), string(v7))
+
+	println(uint8SliceSrc[0])
+	println(uint8SliceDst[0])
+	println(intSliceSrc[0])
+	println(intSliceDst[0])
 }
 
 type (
@@ -30,4 +35,17 @@ var (
 	v5 = map[string]int{}
 	v6 = float64(v1) < 2.6
 	v7 = []byte("foo")
+
+	uint8SliceSrc = []uint8{3, 100}
+	uint8SliceDst []uint8
+	intSliceSrc = []int16{5, 123, 1024}
+	intSliceDst []int16
 )
+
+func init() {
+	uint8SliceDst = make([]uint8, len(uint8SliceSrc))
+	copy(uint8SliceDst, uint8SliceSrc)
+
+	intSliceDst = make([]int16, len(intSliceSrc))
+	copy(intSliceDst, intSliceSrc)
+}

testdata/init.txt

@@ -7,3 +7,7 @@ v4: 0 true
 v5: 0 false
 v6: false
 v7: 3 foo
+3
+3
+5
+5