Fix strict provenance use of pointers

[jamesmunns]

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[jamesmunns]

Adding miri failure output in case someone searches for it in the future:

error: Undefined Behavior: attempting a write access using <22282> at alloc4545[0x0], but that tag does not exist in the borrow stack for this location
   --> /Users/will/.cargo/registry/src/index.crates.io-6f17d22bba15001f/postcard-1.0.4/src/ser/flavors.rs:179:17
    |
179 | ...   core::ptr::copy_nonoverlapping(b.as_ptr(), self.cursor, blen);
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |       |
    |       attempting a write access using <22282> at alloc4545[0x0], but that tag does not exist in the borrow stack for this location
    |       this error occurs as part of an access at alloc4545[0x0..0x1]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <22282> was created by a SharedReadWrite retag at offsets [0x0..0x1e0]
   --> examples/main.rs:20:25
    |
20  | ...ice = postcard::to_slice(data, &mut self.data[ix.to_usize()])?;
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: <22282> was later invalidated at offsets [0x0..0x1e0] by a Unique function-entry retag inside this call
   --> examples/main.rs:20:25
    |
20  | ...ice = postcard::to_slice(data, &mut self.data[ix.to_usize()])?;
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ```