devel/string_theory: update to 3.4

From ChangeLog: https://github.com/zrax/string_theory/releases/tag/3.4

• Fix ST_WCHAR_LITERAL, ST_UTF16_LITERAL and ST_UTF32_LITERAL macros, which would previously create an incorrectly-sized buffer for non-empty strings.
• Add ST::buffer<T>::clear() and ST::string::clear() methods
• Deprecate ST::null and ST::null_t in favor of better alternatives: Construction: Use {} or an explicit constructor (ST::string(), ST::char_buffer()) Comparison: Use .empty() Assignment: Use .clear() or assign a default-constructed object.

Reported by: portscout