Invoke result extended lambdas support

[gevtushenko]

This PR addresses the following issue. The implementation of result_of and invoke_result was complimented with a static assert omitted in device code or compilers different than nvcc. In order not to force users to rewrite device lambdas as function objects, cuda::proclaim_return_type was added. Since the main use case for cuda::proclaim_return_type is device lambdas (which type is unknown in any way) I decided to hide the actual wrapper type in detailed namespace.

Questions:

1. docs/serve failed to start locally without adding gem "webrick" dependency. I'm not sure if this has to be merged.
2. I've just learned about lit and I'm not sure if there's a way to check that the fail test emits text in static asserts. Please advise me on this matter if there's a portable way to do so.

[jrhemstad]

2. I've just learned about lit and I'm not sure if there's a way to check that the fail test emits text in static asserts. Please advise me on this matter if there's a portable way to do so.

Not that I'm aware of. @griwes or @wmaxey may know better than me.

Adding the .fail to the file "extension" is all I've ever done.