Support double-precision floating point

[kpet]

clspv generally doesn't have support for double-precision floating point. The following at least are missing:

• [x] support double literals (see #157)
• [x] support double constants
• [ ] add support for double to the built-in functions
• [ ] support double with operators and add tests
• [ ] update the documentation
• [ ] anything else?

[Trass3r]

add support for double to the built-in functions

Do they need to be emulated? Currently some are mapped to ExtInsts (like here) which are not specified for fp64: https://www.khronos.org/registry/spir-v/specs/unified1/GLSL.std.450.html#_extended_instructions