Differentiate no-std from alloc within tests/no-std

Fixes `no-std` builds for packages which intended to be `no-std` (without
`alloc`).

Updates a variety of MSRVs to 1.73 due to `flexible-transcript` no longer using
`std-shims` to achieve 1.66 (as `std-shims` requires `alloc`). A future
improvement would be for `std-shims` to have an `alloc` feature and only
provide MSRV shims without it.

processor/monero/Cargo.toml

@@ -8,7 +8,6 @@ authors = ["Luke Parker <lukeparker5132@gmail.com>"]
 keywords = []
 edition = "2021"
 publish = false
-rust-version = "1.56"
 
 [package.metadata.docs.rs]
 all-features = true

processor/view-keys/Cargo.toml

@@ -7,7 +7,7 @@ repository = "https://github.com/serai-dex/serai/tree/develop/processor/view-key
 authors = ["Luke Parker <lukeparker5132@gmail.com>"]
 keywords = []
 edition = "2021"
-rust-version = "1.66"
+rust-version = "1.73"
 
 [package.metadata.docs.rs]
 all-features = true