lib/table: remove nonsensical const, add pure

lib/table: remove nonsensical const, add pure

Passing the struct route_table *ptr as const doesn't really help; if
anything it semantically would imply that the returned route_node is
const too since constness should propagate (but it doesn't in C.)

The right thing to do here - which actually helps the compiler optimize
the code too - is to tag functions with __attribute__((pure)).  The
compiler does this automatically if it has the function body (and the
body of all called functions) available.  That should cover most "static
inline" functions in headers, as well as functions in the same file.

However, this doesn't work (at least without LTO) for extern functions.
Hence, add "ext_pure" for this case.  (Built-in "extern" to make lines
shorter.)

Signed-off-by: David Lamparter <equinox@diac24.net>