help updateing go to 1.13

[Jack Hill]

Hi Guix,

I thought I would try my hand at updating our go package to 1.13. Looking 
at the release notes [0] it didn't look to bad. What I have so far can be 
found in the attached patch.

With the patch, go builds. To spot check that everything still looked 
okay, I also tried to build syncthing. Doing so, however, failed when it 
tried to fetch a dependency from the network:

```
go: github.com/AudriusButkevicius/go-nat-pmp@v0.0.0-20160522074932-452c97607362: 
Get 
https://proxy.golang.org/github.com/%21audrius%21butkevicius/go-nat-pmp/@v/v0.0.0-20160522074932-452c97607362.mod:
 dial tcp: lookup proxy.golang.org on [::1]:53: read udp [::1]:35040->[::1]:53: 
read: connection refused
command "go" "run" "build.go" "-no-upgrade" failed with status 1
builder for `/gnu/store/5zs0yrcvr0vwsashsn2i5vx1fgynhmaw-syncthing-1.2.2.drv' 
failed with exit code 1
```

go-github-com-audriusbutkevicius-go-nat-pmp is listed as an input for 
syncthing. I don't know why the change in compiler version would have 
caused this. Thoughts?

[0] https://golang.org/doc/go1.13

[1] On thing that might need addition thinking is that `go get` in 1.13 
now uses the Google mirror for fetching modules and cheksums which is 
different than current behavior. I'm not sure if this poses any privacy 
concerns for the user, and would like to hear what other's think, but this 
is not the main problem I'm struggling with today.

Best,
Jack

0001-WIP-gnu-go-update-to-1.13.patch
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