cmake: Always treat warnings as errors

Enable cast warnings in gcc and clang and always treat warnings as
errors.

GetWordCount now returns std::size_t for simplicity and the word count
is asserted and casted in WordCount (now called CalculateTotalWords.

Silence warnings.

src/literal_string.cpp

@@ -19,8 +19,8 @@ void LiteralString::Fetch(Stream& stream) const {
     stream.Write(string);
 }
 
-u16 LiteralString::GetWordCount() const noexcept {
-    return static_cast<u16>(string.size() / 4 + 1);
+std::size_t LiteralString::GetWordCount() const noexcept {
+    return string.size() / 4 + 1;
 }
 
 bool LiteralString::operator==(const Operand& other) const noexcept {