Pointer arithmetic

[dgrunwald]

The new decompiler ILSpy 3 is lacking support for pointer arithmetic.
The code is instead decompiled into arithmetic on IntPtr, which requires a whole mess of casts (#851):

return (char)(*(ushort*)((long)ptr + (long)(System.IntPtr)(void*)((long)(int)num * 2L)));

This PR introduces support for pointer arithmetic to the expression builder.

• *ptr
• *ptr + int
• Test ptr + long, ptr - long
• Test (byte*)ptr - (byte*)ptr
• Test pointer arithmetic with pointer-to-struct
• Allow pointer comparisons without casting to IntPtr
• Foreach over pointer array: needs Add foreach over array pattern. #946
• Pointer arithmetic compound assignments -> Pointer arithmetic compound assignments #947
• fixed() statement
• Fixed size buffers (unsafe fixed int array[3];)
• stackalloc[]