Lazily load dynamic linq types

[verdie-g]

Partially fixes #939.

I found two code paths that requires iterating on all the types of the world:

1. KeywordsHelper.TryGetValue
2. MethodFinder.FindMethod

In both cases, the "custom types" are not used first so there is a chance that they are never used and don't need to be loaded.

[verdie-g]

@StefH what do you think of that PR?

[StefH]

I still need to check it in detail...