Issue Description:

ManagedBufferPointer has a header property that provides quick access to the header. Unfortunately its setter requires the pointer itself to be mutable, even though no other usage of this type (including withUnsafeMutablePointerToHeader(_:)) has this requirement, and the type itself is supposed to act as a pointer. This seems like an oversight.

class Buffer {}
let ptr = ManagedBufferPointer<Int,Int>(bufferClass: Buffer.self, minimumCapacity: 5, makingHeaderWith: { (_,_) in 0 })
// this works
ptr.withUnsafeMutablePointerToHeader { $0.pointee = 1 }
// this won't compile unless ptr is turned into var
ptr.header = 1