Remove lifetimes from principles

Lifetime checking is out of scope for this library at this time.

Heap lifetime visibility can be achieved through PartitionAlloc, and an application may choose to use smart pointers that trap on lifetime errors, in tests or in production.

If we include smart pointers here, we should hook into PartitionAlloc or otherwise watch lifetimes, but even that only helps with individually-allocated heap objects. Stack and array objects don't get observed.

There's room for on-stack smart pointers, or static analysis such as a borrow checker to improve things here. If such tools become available, we will integrate any needed support into Subspace and eliminate or extend C++ differences from Rust APIs where needed to support them.

PRINCIPLES.md

@@ -132,7 +132,6 @@ This library is an experiment and not intended for use. See the
    be packed into the tail padding, if it exists and is usable by the compiler
    implementation.
 1. Bounds are always checked unless you explicitly ask for them to not be.
-1. Lifetimes are always checked unless you explicitly ask for them to not be.
 1. Small headers. C++ compilation speed is proportional to the amount of input.
     * We will split types into separate headers as much as possible.
     * We may provide headers which group together smaller headers for simplicity