Update to rustc 1.31.0 and rust 2018

[hayd]

cargo check succeeds, but tools/build.py errors out.

[ry]

Cool! But let's wait until #1296 is done.

[hayd]

Now cargo build succeeds, but tools/build.py errors out. 😄
Is build.py setting the rust version somehow?

[hayd]

It seems like the final stage for this PR is to add args += ["--edition=2018"] in rust.gni BUT only on the deno executable not every crate (since not every crate is edition=2018). Any gn tips for this?

[hayd]

Passing an edition flag in gn seems to do the job here, see last commit.

[hayd]

RE appveyor fail, I'm not sure how to update rustc... cc @piscisaureus

[hayd]

I guess I'll try a few changes to the "rustc exists" appveyor check https://github.com/denoland/deno/pull/1306/files#diff-11c909939117928998b102a1fff7d363L302

I want it to be something like:

- if (-not (Test-Path $env:CARGO_HOME)) {
+ if (-not $(rustc --version | grep 1.31.0)) {

[hayd]

I'm not sure what to pass to appveyor in order to bump the version.

Happy to rebase if anyone works out what to put there, or we can bump to 1.31 in another PR then apply rust-2018.

[ry]

@hayd I'm just closing old PRs. We'd like to upgrade still, of course, but this isn't green. Happy to have a new PR in this direction.

[hayd]

@ry yep, this isn't green because I can't bump the rustc version to 1.31 on appveyor.
Once that is updated this is good to go, but it can be another PR.