assert() would not abort in release builds, meaning an OOM condition
would be undetected.

Some escape sequences (in particular, OSC 52 paste responses) can be
very large, even unbounded in length. These can easily overflow
termkey's internal buffer. In order to process these long sequences,
dynamically grow termkey's internal buffer.

FUNC_ATTR_* should only be used in .c files with generated headers.
Defining FUNC_ATTR_* as empty in headers causes misuses of them to be
silently ignored. Instead don't define them by default, and only define
them as empty after a .c file has included its generated header.