最大值uint8 : 0 to 255
uint16 : 0 to 65535
uint32 : 0 to 4294967295
uint 64: to 1844674407370951615
int8 : -128 to 127
int16 : -32768 to 32767
int 32:-2147483648 to 2147483647
int 64:-9223372036854775808 to 9223372036854775807
math包例如是： math.MaxInt64，结果是9223372036854775807