Added InterleaveFields source generator.

[MoFtZ]

This PR adds support for generating a structure of arrays from a struct.

When the [GeneratedStructutreOfArrays] attribute is added to a partial struct, the source generator will populate the field members.

// Example:
public struct MyPoint
{
    public int X;
    public int Y;
}

[GeneratedStructureOfArrays(typeof(MyPoint), 4)]
partial struct MyPoint4
{ }

// Will generate:
unsafe partial struct MyPoint4
{
    public fixed int X[4];
    public fixed int Y[4];
}

The source generator is also able to expand inner structures as necessary.

// Example:
public struct MyPoint
{
    public int X;
    public int Y;
    public InnerStruct Z;
}

public unsafe struct InnerStruct
{
    public fixed int A[4];
    public fixed int B[2];
}

[GeneratedStructureOfArrays(typeof(MyPoint), 4)]
partial struct MyPoint4
{ }

// Will generate:
unsafe partial struct MyPoint4
{
    public fixed int X[4];
    public fixed int Y[4];
    public fixed int Z_A[16];
    public fixed int Z_B[8];
}

[MoFtZ]

Converted to draft, until we can decide on a better name for the marker attribute.

[Yey007]

@MoFtZ Did we come to a conclusion on this? Are we going with GeneratedInterleavingStruct as you had suggested a couple of meetings ago? Or perhaps Interleaved or InterleavedStructure?

[MoFtZ]

InterleaveFields is the current candidate. But I haven't had the time to refactor the PR yet.