Add CreateInfo for pipelines, require explicit pipeline layout creation

[Rua]

Changelog:

Remove:

Changes to shaders and pipelines:
- `EntryPoint` now owns instead of borrows a reference to the `ShaderModule`.
- Instead of an entry point and specialization constants, pipeline construction now takes a `PipelineShaderStageCreateInfo` structure.
- Specialization constants are now provided with a `HashMap` containing `SpecializationConstant` enum values. The `SpecializationConstants` trait is removed, and `vulkano_shaders` no longer generates structs for specialization constants.
- `GraphicsPipelineBuilder` now has a `stages` method that takes an iterator with all shader stages at once, instead of separate methods for each shader type.
- `GraphicsPipelineBuilder` now requires you to provide `input_assembly_state`, `rasterization_state`, `multisample_state` and `color_blend_state` instead of them having default values. You can still call `default()` to generate default values for each of them.
- `GraphicsPipelineBuilder::vertex_input_state` now requires a `VertexInputState` object directly, instead of a `VertexDefinition`. The `VertexDefinition` trait can be used to create the object.

Replace with:

Changes to pipeline construction:
- Pipelines are now constructed with the standard complement of `new`, `new_unchecked` and `from_handle` constructors, that each take a `ComputePipelineCreateInfo` or `GraphicsPipelineCreateInfo` struct.
- Pipeline layouts are no longer automatically constructed when creating a pipeline. You can auto-create a pipeline layout (and modify the auto-generated parameters if needed) using the new `PipelineDescriptorSetLayoutCreateInfo` type.
- `GraphicsPipelineCreateInfo::vertex_input_state` requires a `VertexInputState` struct directly, instead of a `VertexDefinition`. The `VertexDefinition` trait can be used to create the struct.
- `GraphicsPipelineCreateInfo` now requires you to provide `input_assembly_state`, `rasterization_state`, `multisample_state` and `color_blend_state` instead of them having default values. You can still call `default()` to generate default values for each of them.
- Instead of an entry point and specialization constants, pipeline construction now takes a `PipelineShaderStageCreateInfo` structure. `GraphicsPipelineCreateInfo` has a `stages` member that takes all shader stages at once, instead of separate members for each shader type. `EntryPoint` now owns instead of borrows a reference to the `ShaderModule`, so this is easier.
- Specialization constants are now provided with a `HashMap` containing `SpecializationConstant` enum values. The `SpecializationConstants` trait is removed, and `vulkano_shaders` no longer generates structs for specialization constants.

This brings pipeline construction in line with other object types. Pipeline layout construction is no longer automatic in pipeline construction, but I made a new type to make it easier to auto-construct them separately.

[marc0246]

Very nice, again one step closer to Vulkan with the pipeline create infos!