Bump ssz_types to v0.12.2 (#8032)

https://github.com/sigp/lighthouse/issues/8012


  Replace all instances of `VariableList::from` and `FixedVector::from` to their `try_from` variants.

While I tried to use proper error handling in most cases, there were certain situations where adding an `expect` for situations where `try_from` can trivially never fail avoided adding a lot of extra complexity.


Co-Authored-By: Mac L <mjladson@pm.me>

Co-Authored-By: Michael Sproul <michaelsproul@users.noreply.github.com>

Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

beacon_node/lighthouse_network/src/rpc/methods.rs

@@ -29,15 +29,21 @@ pub const MAX_ERROR_LEN: u64 = 256;
 #[derive(Debug, Clone)]
 pub struct ErrorType(pub VariableList<u8, MaxErrorLen>);
 
-impl From<String> for ErrorType {
-    fn from(s: String) -> Self {
-        Self(VariableList::from(s.as_bytes().to_vec()))
+impl From<&str> for ErrorType {
+    // This will truncate the error if `string.as_bytes()` exceeds `MaxErrorLen`.
+    fn from(s: &str) -> Self {
+        let mut bytes = s.as_bytes().to_vec();
+        bytes.truncate(MAX_ERROR_LEN as usize);
+        Self(
+            VariableList::try_from(bytes)
+                .expect("length should not exceed MaxErrorLen after truncation"),
+        )
     }
 }
 
-impl From<&str> for ErrorType {
-    fn from(s: &str) -> Self {
-        Self(VariableList::from(s.as_bytes().to_vec()))
+impl From<String> for ErrorType {
+    fn from(s: String) -> Self {
+        Self::from(s.as_str())
     }
 }