bench: add memory benchmarks

[rgrinberg]

$ dune exec benchmarks/memory.exe will demonstrate the pathological example

[vouillon]

The automata remembers the last size + 1 characters, which takes an exponential amount of memory.

In this case, one only needs to check that there are size zeroes or ones after the first 1, which would only require a linear amount of memory. For that, when building the automaton one would need to see that repn (set "01") (n + 1) (Some (n + 1) is subsumed by repn (set "01") n (Some n).

But this is very fragile. For a longest match semantics or a first match / greedy semantics, one really needs to remember all these characters. And if we change the regular expression only a little bit, like below, one cannot avoid the exponential behavior either.

  seq
    [ rep (set "01")
    ; char '1'
    ; repn (set "01") size (Some size)
    ; char 'x'
    ]

DFAs are not good at counting...