feat: Reuse typescript program in between parses

[denieler]

Because we are not reusing the same Typescript instance while parsing ts files there are this issue styleguidist/react-docgen-typescript#112 and this issue #37.

So here I've created a code, which reuse the same program in between parses in the same way as in ts-loader it's done (https://github.com/TypeStrong/ts-loader/blob/master/src/servicesHost.ts)

[denieler]

@strothj please review

[strothj]

Awesome. I'll have some time to review this in more detail later today (Sunday).

I think one of the original concerns I had when working on this issue was about potential issues that could occur when using a multi-threaded build (https://github.com/TypeStrong/ts-loader#parallelising-builds). I'm not sure that's a valid concern anyways since the ts-loader docs now mention there's less benefit to doing that.

[denieler]

@strothj hey! :) how is it going? :) can we merge it?

[strothj]

Thanks for the pull request! I've published this as version 3.1.0. Let me know if you have any issues and sorry for the delay.

[zoltanbedi]

While it does speed up the build I noticed that it is not working with the latest typescript (3.4.1) 😔
false alarm. it has nothing to do with this package. the problem is in styled components. see: microsoft/TypeScript#30663

[strann]

@strothj can this change be reflected in the underyling react-docgen-typescript package itself? We are using it directly in a node script and are seeing some very slow compile times as our package grows.