Refactor str_buffer, add tests for it.

• Add documentation to functions in str_buffer.
• Add SBUFsubstToken to substitute tokens in the string buffer.
• Fix the string buffer overflowing when instantiated with a size of 0.
• Fix SBUFflush not setting a null byte, causing functions like SBUF2str to use the previous contents instead of an empty string.
• Fix format string encoding errors silently leading to a memory overflow instead of crashing with a useful message.
  • Change CTIerrorInternal to not use PrintMessage to avoid a scenario where errors could potentially occur during error handling.
• Some debug messages about buffer size now display before attempting to increase it, making debugging easier.