secrecy v0.10.0

[tony-iqlusion]

This release represents a significant redesign of the secrecy crate.

The most notable change is the generic Secret<T> type has been removed: instead use SecretBox<T>
which stores secrets on the heap instead of the stack. Many of the other changes fall out of this
change and things which were previously type aliases of Secret<T> are now type aliases of
SecretBox<T>.

This unfortunately means this crate no longer has support for "heapless" no_std targets. We don't
have a good solution for these targets, which was a motivation for this change in the first place.

Added

• SecretBox::{init_with, try_init_with} (#1212)
• SecretBox::init_with_mut (#1213)
• ?Sized bounds for SecretBox (#1213)
• SecretSlice<T> (#1214)

Changed

• Rust 2021 edition upgrade (#889)
• MSRV 1.60 (#1105)
• SecretBox<T> is now a newtype rather than a type alias of `Secret<Box> (#1140)
• SecretString is now a type alias for SecretBox<str> (#1213)
• Disable serde default features (#1194)

Removed

• alloc feature: now a hard dependency (#1140)
• bytes crate integration: no replacement (#1140)
• DebugSecret trait: no replacement (#1140)
• Secret<T>: use SecretBox<T> instead (#1140)