add int max min consts

vlib/builtin/js/int.js.v

@@ -2,6 +2,36 @@ module builtin
 
 type byte = u8
 
+pub const min_i8 = i8(-128)
+pub const max_i8 = i8(127)
+
+pub const min_i16 = i16(-32768)
+pub const max_i16 = i16(32767)
+
+pub const min_i32 = i32(-2147483648)
+pub const max_i32 = i32(2147483647)
+
+pub const min_int = min_i32
+pub const max_int = max_i32
+
+// -9223372036854775808 is wrong, because C compilers parse literal values
+// without sign first, and 9223372036854775808 overflows i64, hence the
+// consecutive subtraction by 1
+pub const min_i64 = i64(-9223372036854775807 - 1)
+pub const max_i64 = i64(9223372036854775807)
+
+pub const min_u8 = u8(0)
+pub const max_u8 = u8(255)
+
+pub const min_u16 = u16(0)
+pub const max_u16 = u16(65535)
+
+pub const min_u32 = u32(0)
+pub const max_u32 = u32(4294967295)
+
+pub const min_u64 = u64(0)
+pub const max_u64 = u64(18446744073709551615)
+
 pub fn (i i8) str() string {
 	mut res := ''
 	#res.str = i.val.toString()