package main

import (
    "encoding/binary"
    "fmt"
)

func main() {
    u8 := []uint8{0, 1, 2, 3}
    u32LE := binary.LittleEndian.Uint32(u8)
    fmt.Println("little-endian:", u8, "to", u32LE)
    u32BE := binary.BigEndian.Uint32(u8)
    fmt.Println("big-endian:   ", u8, "to", u32BE)
}

Output:

little-endian: [0 1 2 3] to 50462976
big-endian:    [0 1 2 3] to 66051

The Go binary package functions are implemented as a series of shifts.

func (littleEndian) Uint32(b []byte) uint32 {
    return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
}

func (bigEndian) Uint32(b []byte) uint32 {
    return uint32(b[3]) | uint32(b[2])<<8 | uint32(b[1])<<16 | uint32(b[0])<<24
}

Simply convert it :

fmt.Println(strconv.Itoa(int(str[1])))

The predeclared Go int type size is implementation-specific, either 32 or 64 bits (Numeric types).

Here's an example of converting big-endian ints to bytes (uint8s).

package main

import (
    "encoding/binary"
    "fmt"
    "reflect"
)

func IntsToBytesBE(i []int) []byte {
    intSize := int(reflect.TypeOf(i).Elem().Size())
    b := make([]byte, intSize*len(i))
    for n, s := range i {
        switch intSize {
        case 64 / 8:
            binary.BigEndian.PutUint64(b[intSize*n:], uint64(s))
        case 32 / 8:
            binary.BigEndian.PutUint32(b[intSize*n:], uint32(s))
        default:
            panic("unreachable")
        }
    }
    return b
}

func main() {
    i := []int{0, 1, 2, 3}
    fmt.Println("int size:", int(reflect.TypeOf(i[0]).Size()), "bytes")
    fmt.Println("ints:", i)
    fmt.Println("bytes:", IntsToBytesBE(i))
}

Output:

int size: 4 bytes
ints: [0 1 2 3]
bytes: [0 0 0 0 0 0 0 1 0 0 0 2 0 0 0 3]

or

int size: 8 bytes
ints: [0 1 2 3]
bytes: [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0 3]