parser: fix quadratic pointer chasing

parser: fix quadratic pointer chasing

In the AST, lists (e.g. the list of statements in a file) are kept in
singly-linked lists -- each AST node has a `next` pointer available for
this purpose.

Previously, a node was added to the list by starting from the head,
chasing to the last, and appending. So creating a list of length N would
take ~N^2/2 pointer dereferences.

Now, we always (temporarily) keep the last as well, so appending is O(1)
instead of O(N).

Given a keymap

    xkb_keymap {
    xkb_keycodes {
    minimum = 8;
    minimum = 8;
    minimum = 8;
    minimum = 8;
    minimum = 8;
    [... repeated N times ...]
    };
    xkb_types {};
    xkb_compat {};
    xkb_symbols {};
    };

The compilation times are

N       | Before   | After
--------|----------|-------
10,000  | 0.407s   | 0.006s
20,000  | 1.851s   | 0.015s
30,000  | 5.737s   | 0.021s
40,000  | 12.759s  | 0.023s
50,000  | 21.489s  | 0.035s
60,000  | 40.473s  | 0.041s
70,000  | 53.336s  | 0.039s
80,000  | 72.485s  | 0.044s
90,000  | 94.703s  | 0.048s
100,000 | 118.390s | 0.057s

Another option is to ditch the linked lists and use arrays instead. I
got it to work, but its more involved and allocation heavy so turns out
to be worse without further optimizations.

Signed-off-by: Ran Benita <ran@unusedvar.com>