[release/1.4.7] accounts/abi: Negative numbers not properly converted in ABI encoding

When converting a negative number e.g., -2, the resulting ABI encoding
should look as follows:
fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffe.
However, since the check of the type is for an uint instead of an
int, it results in the following ABI encoding:
0101010101010101010101010101010101010101010101010101010101010102. The
Ethereum ABI
(https://github.com/ethereum/wiki/wiki/Ethereum-Contract-ABI) says,
that signed integers are stored in two's complement which should be
of the form ffffff.... and not 01010101..... for e.g. -1. Thus, I
removed the type check in numbers.go as well as the function S256
as I don't think they are correct. Or maybe I'm missing something?

(cherry picked from commit 89c6c5bb85)

accounts/abi/packing.go

@@ -25,7 +25,7 @@ import (
 // packBytesSlice packs the given bytes as [L, V] as the canonical representation
 // bytes slice
 func packBytesSlice(bytes []byte, l int) []byte {
-	len := packNum(reflect.ValueOf(l), UintTy)
+	len := packNum(reflect.ValueOf(l))
 	return append(len, common.RightPadBytes(bytes, (l+31)/32*32)...)
 }
 
@@ -34,7 +34,7 @@ func packBytesSlice(bytes []byte, l int) []byte {
 func packElement(t Type, reflectValue reflect.Value) []byte {
 	switch t.T {
 	case IntTy, UintTy:
-		return packNum(reflectValue, t.T)
+		return packNum(reflectValue)
 	case StringTy:
 		return packBytesSlice([]byte(reflectValue.String()), reflectValue.Len())
 	case AddressTy: