Wrap the maze in walls

Fixes #4 and totally messes up #7 

We currently use size in 10 functions, but most of those functions will work just fine without size if we wrap the maze in walls. @r00k proposed this super amazing idea in #4. :rainbow: 

This PR [adds some outer walls](https://github.com/clojure-rica/maize/blob/a3c2ea99993580273c843380812d1c99580ef941/src-cljs/maze/core.cljs#L76) to `generate-maze` so we don't need size in so many places.

There are now [a total of three functions](https://github.com/clojure-rica/maize/blob/a3c2ea99993580273c843380812d1c99580ef941/src-cljs/maze/core.cljs#L64-73) that need size and all three are only referenced from the outer-most [`generate-maze`](https://github.com/clojure-rica/maize/blob/a3c2ea99993580273c843380812d1c99580ef941/src-cljs/maze/core.cljs#L76-77) and [`solve-maze`](https://github.com/clojure-rica/maize/blob/a3c2ea99993580273c843380812d1c99580ef941/src-cljs/maze/core.cljs#L80)