Make pieces::Stack methods private.

[porcuquine]

The pieces::Stack struct is only used in the implementation of compute_comm_d, and accurate usage of its helper methods is required for correctness. For example, peek() and peek2 must not be called when the stack is not large enough. This correctness is guaranteed by usage of Stack as a helper data structure within compute_comm_d as written, but there is no reason to expose these methods to callers who might misuse them. Put differently, correctness of the Stack data structure depends on these methods being private. This allows a reader to determine — by only inspecting the pieces module — that these methods are never called incorrectly.

[cryptonemo]

Looks good pending CI

[porcuquine]

This is a further response to Security Audit FPS-11.