Overhaul of scala.scalanative.unsafe.{stackalloc,alloc}

[WojciechMazur]

Deprecation and further removal of stackalloc/alloc methods taking a primitive Int was a mistake - it made the Scala Native code less readable and did not improved typesafety in any way (-1.toUnsigned = 0xFFFFFFF...). In this PR we revert some of this changes, and provide new mechanisms for safer interop.

• Restore stackalloc[T](Int) and alloc[T](Int) methods. The now have special handling for literals integers and detect zero and negative literals at compile time. Non-literal values now have a runtime check to ensure that provide integer is positive integer. Unsigned variants are still present and have the same sementics as before.
• Intrinsic.stackallocsignature has changed, methods take now generic T type used to resolve size of required memory. The size: RawSize argument no longer describes size of the allocated memory, instead it describe number of elements of sizeOf[T] to allocate. This change allows to skip some arithmetic operations at runtime/optimizer and allows to use the nir.Type argument of nir.Op.Stackalloc (previously always Byte). This work is based on 2-parameter (num/size) stackalloc intrinsic overload #3260
• 0-initialization of stackallocated memory is now moved to Lowering stage. Thanks to having a full information about allocated type, we can calculate exactly the required amount of memory at linktime after optimizer run, and allows to add optimizer runs designed to removal of zero-initialization of manually initialized memory.
• All special intrinsic operations used as intermiediete step of compilation were moved to Intrinsics.internal object
• Removed most of no longer required toUnsigned conversion

[ekrich]

This looks like it will really help, especially with some math code I have.