fix incorrect use of toSszType (#82)

* also fix missing `fromSszBytes`

ssz_serialization.nim

@@ -271,8 +271,8 @@ proc writeValue*[T](
 
 proc readValue*(
     r: var SszReader, val: var auto) {.raises: [SszError, IOError].} =
-  mixin readSszBytes
-  type T = typeof val
+  mixin readSszBytes, toSszType
+  type T = typeof toSszType(val)
   when isFixedSize(T):
     const minimalSize = fixedPortionSize(T)
     if r.stream.readable(minimalSize):
@@ -289,13 +289,4 @@ proc readSszBytes*[T](
   # Overload `readSszBytes` to perform custom operations on T after
   # deserialization
   mixin readSszValue
-
-  when T isnot SszType:
-    mixin toSszType
-    readSszValue(data, toSszType(val))
-  else:
-    # Although inside toSszType already have
-    # compile time check for SszType,
-    # somehow it will trigger `dereferencing pointer to incomplete type`
-    # error on linux gcc.
-    readSszValue(data, val)
+  readSszValue(data, val)

ssz_serialization/codec.nim

@@ -250,7 +250,9 @@ proc readSszValue*[T](
         T, "list element offset points past the end of the input")
     int(offset)
 
-  when val is BitList:
+  when compiles(fromSszBytes(T, input)):
+    val = fromSszBytes(T, input)
+  elif val is BitList:
     if input.len == 0:
       raiseMalformedSszError(T, "invalid empty value")
 
@@ -439,9 +441,6 @@ proc readSszValue*[T](
       else:
         val = Opt.some(v)
 
-  elif val is UintN|bool:
-    val = fromSszBytes(T, input)
-
   elif val is BitArray:
     if sizeof(val) != input.len:
       raiseIncorrectSize(T)