Fix to_bytes for numeric types (#447)

make to_bytes use mem::size_of instead of hardcoded length value for numeric types

fuel-types/src/numeric_types.rs

@@ -35,14 +35,16 @@ macro_rules! key {
 
 macro_rules! key_methods {
     ($i:ident, $t:ty) => {
+        const SIZE: usize = core::mem::size_of::<$t>();
+
         impl $i {
             /// Number constructor.
             pub const fn new(number: $t) -> Self {
                 Self(number)
             }
 
             /// Convert to array of big endian bytes.
-            pub fn to_bytes(self) -> [u8; 4] {
+            pub fn to_bytes(self) -> [u8; SIZE] {
                 self.0.to_be_bytes()
             }
 
@@ -57,8 +59,6 @@ macro_rules! key_methods {
             }
         }
 
-        const SIZE: usize = core::mem::size_of::<$t>();
-
         #[cfg(feature = "random")]
         impl rand::Fill for $i {
             fn try_fill<R: rand::Rng + ?Sized>(&mut self, rng: &mut R) -> Result<(), rand::Error> {