Optimize string single character append

Now uses a specialized implementation instead of the append multiple
characters implementation. Useful for appending single characters to
string streams (ex. newline).

src/string.cpp

@@ -255,7 +255,12 @@ istring& istring::operator+=(const string_view& str) {
 }
 
 istring& istring::operator+=(char ch) {
-	return append(1, ch);
+	if (m_size < m_capacity) {
+		m_str[m_size] = ch;
+		++m_size;
+	}
+	
+	return *this;
 }
 
 istring& istring::replace(size_t pos, size_t count, const string_view& str) {