fix: parse bbox into expr

[gadomski]

Closes #245

Command-line check:

$ cargo run -- -o sql " s_overlaps(geom,BBox(9.978199, 53.541309, 10.010294, 53.557241))"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.10s
     Running `target/debug/cql2 -o sql ' s_overlaps(geom,BBox(9.978199, 53.541309, 10.010294, 53.557241))'`
st_overlaps(geom, st_makeenvelope(9.978199, 53.541309, 10.010294, 53.557241))