Catch typos like `1_32` instead of `1_i32`

I missed a letter in a very subtle way that still compiled, so it might be a good candidate for linting:
```rust
let mut x = 1_32;
```
when I meant to type
```rust
let mut x = 1_i32;
```
This, of course, made it do something _very_ different from what I wanted...