Add list and custom type support in the style of ppx_mysql

[anmonteiro]

This diff does a few things:

1. ports from core to base which is much lighter weight
2. Adds list support in the input parameters
3. As part of 2. a new sub-library is added which makes this PPX have a small runtime for dynamic parameters in the style of https://github.com/paurkedal/ocaml-caqti/issues/15#issuecomment-386793041

There are a couple things left to do here:

• add some form of documentation to the README that explains how to use list input parameters
• more general than this PR, but the lib would benefit from a refactoring pass, there's some things that are hard to work with.

Note: because queries containing list parameters are dynamically constructed, they need to be ~oneshot:true as per Caqti's docs.

[roddyyaga]

Thanks, this is great. I'll take a closer look tomorrow.

[anmonteiro]

Just added support for custom types too, and I'm gonna work on adding some docs to the README.

I tried to separate this PR into different relevant commits, so while it's become a little large, it's still best reviewed commit by commit. Let me know if you want me to open different PRs instead.

[roddyyaga]

Single PR is fine; everything looks good. Is there anything else you want to add to this PR? Will merge if not.

[mxmzdlv]

It seems that new base types (e.g. ptime) don't work:

Fatal error: exception Error in ppx_rapper: Unknown type specification 'ptime'.

I think new types need to be added to build_param function in query.mll

[anmonteiro]

oh good point, I'll fix later

[anmonteiro]

I just fixed the support for ptime and all the other added types. Thanks again for pointing it out, @mxmzdlv