Supports SSL certificate checking.

This PR

* Compiles a CA cert chain from NSS into our runtime
* Fixes SSL context creation, enables CA checking and loading
* Adds hostname checking to the `require('tls')` module
* Fixes to regex and net modules to handle cert invalidation
* Adds SHA256 support to axTLS (thanks to fortuna, how handy, needed for api.github.com)
* Sets ENABLE_TLS=1
Should be `const` so it goes in `.rodata` in flash. Then you can skip the section override nonsense that doesn't seem to even be used.