fix: remove reliance on invalid decompositions in selector calculation (

#9337)

Decomposing `hash` into `SELECTOR_SIZE` bytes relies on the compiler not
checking the correctness of decompositions performed at compile time.
This means the current code does not work at runtime and we're also
looking to enforce the correctness of this decomposition at
compile-time.

I've replaced this decomposition with a truncation into 32 bits as this
seems equivalent.

noir-projects/noir-protocol-circuits/crates/types/src/abis/event_selector.nr

@@ -1,8 +1,5 @@
-use crate::utils::field::field_from_bytes;
 use crate::traits::{Serialize, Deserialize, FromField, ToField, Empty};
 
-global SELECTOR_SIZE: u32 = 4;
-
 pub struct EventSelector {
     // 1st 4-bytes (big-endian leftmost) of abi-encoding of an event.
     inner: u32,
@@ -53,10 +50,8 @@ impl EventSelector {
         let bytes = signature.as_bytes();
         let hash = crate::hash::poseidon2_hash_bytes(bytes);
 
-        // We choose the last SELECTOR_SIZE bytes of the hash to avoid getting the first byte that is not full
-        let hash_bytes = hash.to_be_bytes::<SELECTOR_SIZE>();
-
-        EventSelector::from_field(field_from_bytes(hash_bytes, true))
+        // `hash` is automatically truncated to fit within 32 bits.
+        EventSelector::from_field(hash)
     }
 
     pub fn zero() -> Self {

noir-projects/noir-protocol-circuits/crates/types/src/abis/function_selector.nr

@@ -1,8 +1,5 @@
-use crate::utils::field::field_from_bytes;
 use crate::traits::{Serialize, Deserialize, FromField, ToField, Empty};
 
-global SELECTOR_SIZE: u32 = 4;
-
 pub struct FunctionSelector {
     // 1st 4-bytes of abi-encoding of function.
     inner: u32,
@@ -53,10 +50,8 @@ impl FunctionSelector {
         let bytes = signature.as_bytes();
         let hash = crate::hash::poseidon2_hash_bytes(bytes);
 
-        // We choose the last SELECTOR_SIZE bytes of the hash to avoid getting the first byte that is not full
-        let hash_bytes = hash.to_be_bytes::<SELECTOR_SIZE>();
-
-        FunctionSelector::from_field(field_from_bytes(hash_bytes, true))
+        // `hash` is automatically truncated to fit within 32 bits.
+        FunctionSelector::from_field(hash)
     }
 
     pub fn zero() -> Self {