ClojureScript self-host port and build tweaks

[arichiardi]

Three main changes, let me know if you would rather have three PRs (not squashing the commits would probably achieve the same result :D) :

• Stop bundling xregexp

  This change makes cuerdas compatible with the latest ClojureScript release: npm dependencies does not need to be bundled and they can be declared by adding a key in deps.cljs - the compiler will see the dep is declared and will try to install in dependent libraries.

  • Make code self-host compatible

    Note that this patch does not port cuerdas.core/istr and cuerdas.core/<<.

  • Add deps.edn and figwheel-main configuration

I also added a small self-host section in the docs.

[arichiardi]

The failure is due to cuerdas.core/capital and fixed in #67

[arichiardi]

Another note, the problem with << and istr lies in silent-read - I need more understanding of the return:

[(read r) (slurp r)]

in particular where the reader stops after read...so that slurp can continue.