FCS should unsubscribe from TP Invalidate events when disposing builders

[dsyme]

See this comment: #158 (comment)

We should unsubscribe from the Invalidate event and/or use a weak reference, to reduce the impact of leaks like this: https://github.com/fsharp/FSharp.Compiler.Service/blob/master/src/fsharp/build.fs#L4091

[quasilord]

I have a fix for this

[quasilord]

Update: This alone doesn't give any protection against memory leaks because the TypeProviderConfig object given to the type provider still captures the FCS resources.

The real solution is that the type provider should dispose of things properly.

[quasilord]

Updated fix published in FCS 0.0.54