update deps

[rizakrko]

Updates:
quote: 0.5 => 0.6
syn: 0.13 => 0.14
proc-macro2: 0.3 => 0.4

[sgrif]

Is there a specific reason we need any of these changes?

[rizakrko]

There is no particular reason, just a routine update

[sgrif]

I'd prefer not to arbitrarily churn code unless there's a specific reason, especially WRT things as unstable as syn and proc_macro2. If someone on the core team is reading and strongly disagrees, feel free to reopen.

[sgrif]

@Eijebong disagrees

[Eijebong]

I don't think they're that unstable and I'd prefer to review 0.13 -> 0.14 and then 0.14 -> whatever than jumping two versions.

[rizakrko]

Failure is related to changes in nightly compiler, so #51042 has to be merged, and then clippy and related nightly needs to be updated.

[sugar700]

If somebody needs a reason, this reduces amount of compilation if using diesel with another crate which has newest version of those dependencies (a specific example - serde).

[Eijebong]

Superseded by #1789