Remove exports for header-only functions/methods

Change the VTKM_CONT_EXPORT to VTKM_CONT. (Likewise for EXEC and EXEC_CONT.) Remove the inline from these macros so that they can be applied to everything, including implementations in a library.

Because inline is not declared in these modifies, you have to add the keyword to functions and methods where the implementation is not inlined in the class.

The intention of this change is two-fold. First, with the introduction of actual libraries, the idea of export is being overloaded to the point of confusion. To alleviate this confusion, we no longer call code defined in a header as "exported." Second, declaring inline everywhere as been causing some issues with some compilers (longer compile times with worse runtime performance). All these inline hints were not helpful.