[io_limbs] handle unmarshal for sources longer, but smaller

If a given source `src` is given to `unmarshal` where the most
significant bytes are all zero beyond the size that fits into the
destination BigInt, we still return a successful parse. Previously we
would just return false whenever we found more bytes than can possibly
fit (this leads to issues with e.g. EVM precompiles tests, which come
as strings that are longer).

Note: We can probably do the all zero check in a better
way (i.e. using the fact that endianness does not matter and just
compare all relevant bytes, but not sure about the most elegant way to
do that, taking the dynamic sizes into account)

constantine/serialization/io_limbs.nim

@@ -63,8 +63,17 @@ func unmarshalLE[T](
 
     # if full, dump
     if acc_len >= wordBitWidth:
-      if dst_idx == dst.len:
-        return false
+      if dst_idx == dst.len: # return, however check if all remaining
+        # elements are 0 (more significant bits beyond `src_idx`)
+        # For example, given a
+        # `src = "DDCCBBAA0000"`
+        # and a 4 byte LittleEndian `BigInt`, `src` is fine, because the most
+        # significant two bytes are all zero.
+        var allZero = true
+        for jidx in src_idx ..< dst.len:
+          if src[jidx] != 0: allZero = false
+        if allZero: return true
+        else: return false
 
       dst[dst_idx] = acc
       inc dst_idx
@@ -118,8 +127,17 @@ func unmarshalBE[T](
 
     # if full, dump
     if acc_len >= wordBitWidth:
-      if dst_idx == dst.len:
-        return false
+      if dst_idx == dst.len: # return, however check if all remaining
+        # elements are 0 (more significant bits beyond `src_idx-1`)
+        # For examplle, given a
+        # `src = "0000AABBCCDD"`
+        # and a 4 byte BigEndian `BigInt`, `src` is fine, because the most
+        # significant two bytes are all zero.
+        var allZero = true
+        for jidx in countdown(src_idx, 0):
+          if src[jidx] != 0: allZero = false
+        if allZero: return true
+        else: return false
 
       dst[dst_idx] = acc
       inc dst_idx