Add get_info<Info>() specializations

[kylelutz]

Right now, the get_info<T>(enum) function allows users to retrieve values from the corresponding clGet*Info() functions.

However, the current API requires users to specify both the enum name (e.g. CL_DEVICE_LOCAL_MEM_SIZE) and the value type (e.g. cl_ulong).

We should add specializations for the known enum types with hard-coded return types. For example, the following test should pass:

device.get_info<CL_DEVICE_LOCAL_MEM_SIZE>() == device.get_info<cl_ulong>(CL_DEVICE_LOCAL_MEM_SIZE);

Care should be taken to ensure that only enum types available from the OpenCL version reported by cl.h are used. For example, don't try to use info enums from OpenCL 1.2 like CL_DEVICE_PRINTF_BUFFER_SIZE if CL_VERSION_1_2 is not defined.

[kylelutz]

Implemented in PR #176