Messages can be very large, so we don't really want to allocate them on the
stack. Switch to using the heap for them, and try to clean up some of the
string handling while we're at it.
Repeat the fix from b10f90d for megolm messages.
It turns out that the 'length' argument to 'Pointer_stringify' doesn't work if
the input includes characters >= 128.
Rather than try to figure out which methods can return UTF-8, and which always
return plain ascii, replace all uses of Pointer_stringify with a 'length'
argument with the version that expects a NULL-terminated input, and extend the
buffer by a byte to allow space for a null-terminator.
In the case of decrypt, we need to add the null ourself.
Fixes https://github.com/vector-im/vector-web/issues/2078.
