Bump our MSRV to 1.45.2

[paolobarbolini]

• Tokio bumped the MSRV to 1.45 on master, so the 0.3 will require Rust 1.45 if not greater
• Other crates we depend on also bumped their MSRV (see clippy: silence matches! warning from 1.47 #481 and ci: skip async-std build on 1.40.0 #462)
• 1.45 was also the version that stabilized function-like procedural macros and expressions, so a lot of crates are probably going to update to it soon

[paolobarbolini]

Well, it looks like tokio 0.3 is going to require >=1.47. Oh well, we'll see how long it takes to get to the final 0.10.0 release and we might bump ours too.

tokio-rs/tokio#2960

[carllerche]

Looks like 0.3 will actually stick to 1.45