Bump guard version to fix box_syntax error

[deejayem]

With the lastest version of the rust compiler, I am unable to build ea. The error is as follows (but I removed the duplicates that only differ by line number):

Checking guard v0.5.1
error: `box_syntax` has been removed
--> /Users/djm/.cargo/registry/src/index.crates.io-6f17d22bba15001f/guard-0.5.1/src/lib.rs:426:20
|
426 |         let foo = (box 42, [1, 2, 3]);
|                    ^^^^^^
|
help: use `Box::new()` instead
|
426 |         let foo = (Box::new(42), [1, 2, 3]);
|                    ~~~~~~~~~~~~
...
error: could not compile `guard` (lib) due to 4 previous errors

This is caused by rust-lang/rust#108471

This is fixed in guard 0.5.2.

I also had to fix a (seemingly) unrelated compilation issue to get it to compile, but I'll raise a separate PR for that.