graph: Add unsigned vs. signed comment to From<U128> for BigInt

Author: Jannis Pohlmann

graph/src/data/store/scalar.rs

@@ -82,6 +82,11 @@ impl From<i32> for BigInt {
 }
 
 impl From<U128> for BigInt {
+    /// This implementation assumes that U128 represents an unsigned U128,
+    /// and not a signed U128 (aka int128 in Solidity). Right now, this is
+    /// all we need (for block numbers). If it ever becomes necessary to
+    /// handle signed U128s, we should add the same
+    /// `{to,from}_{signed,unsigned}_u128` methods that we have for U256.
     fn from(n: U128) -> BigInt {
         let mut bytes: [u8; 16] = [0; 16];
         n.to_little_endian(&mut bytes);