[proposal] Add an origin parameter to `Buffer`

[martinvuyk]

Add an origin parameter to Buffer

Maybe we can do this:

struct Buffer[
    mut: Bool, //, *,
    origin: Origin[mut],
    ... # alignment etc.
](Movable):
    alias _U = UnsafePointer[UInt8, mut=mut, origin=origin] # alignment etc.
    # these two fields could also be replaced by `Span[UInt8, origin]`
    var ptr: Self._U
    var size: Int
    var owns: Bool

    fn __init__(out self, ptr: Self._U, size: Int, owns: Bool = True):
        self.ptr = ptr
        self.size = size
        self.owns = owns

    fn alloc[I: Intable, //, T: DType = DType.uint8](length: I) -> Buffer[origin=MutableAnyOrigin.empty]: ...

origins are kept throughout the code this way which is much safer. It will involve a lot of work though.

[winding-lines]

This is a great idea, like you said a lot of work.

My use case is with Arrow as the interface between Python and Mojo, so data will flow from python to mojo for processing and then back. I think of it more like arena allocation, where all the data will be deallocated at once. So figuring out the origin story is a second term goal.