Prevent inlining big functions

[alfonsogarciacaro]

Follow up of #29. This prevents inlining big functions by putting the bulk of the fetch code in non-inlined __tryFetchAs function that receives the data and response Type arguments. tryFetchAs is still inlined but now the only thing it does is generating the types with typeof and passing them to __tryFetchAs.

Note: it should be possible to mark __tryFetchAs with internal so it's not visible to consumers. But sometimes I get errors from the F# compiler when inlined functions call private/internal functions so for now I'm just prefixing it with __.