Update to Zig 0.15.1 and remove managed secret

[eligrubb]

This PR updates the library's Zig version to 0.15.1. The 0.14.1 can still be found under branches.

The primary break change is, like std.ArrayList, the zecrecy library is moving to a soley unmanaged memory model. All methods that impact memory management now require an allocator as a parameter.

• The Secret interface no longer stores an allocator field.
• The SecretUnmanaged and SecretStringUnmanaged interfaces have been removed.

Additionally, the SecretString interface has been renamed to SecretBytes to better reflect typical use.