Add mrsh_word_append_to

`mrsh_word_append_to(word, buffer)` would write to a buffer. This would allow to optimize when calling `mrsh_word_str` in a loop.

Search the repository for mrsh_word_str and inspect the word-related code that implements it. Trace how words are currently converted in a loop, then add the requested mrsh_word_append_to(word, buffer) entry point so callers can write directly to a buffer and avoid repeated string conversion.

Written by the indexing model from the issue text.