Add go.mod

Code using Go modules currently cannot import any Sia packages. If you try, you'll get this error:

# gitlab.com/NebulousLabs/Sia/persist
../go/pkg/mod/gitlab.com/!nebulous!labs/!sia@v1.4.0/persist/boltdb.go:6:2: imported and not used: "github.com/coreos/bbolt"
../go/pkg/mod/gitlab.com/!nebulous!labs/!sia@v1.4.0/persist/boltdb.go:13:2: undefined: bolt

For whatever reason, Go modules do not seem to play nice with implicit import renames. That is, we're importing bbolt, but the actual package name is bolt, and we don't explicitly rename the import to bolt.

0a541b26 fixes this by renaming all imports of bbolt. However, now we get a new error:

# gitlab.com/NebulousLabs/Sia/modules/consensus
modules/consensus/accept.go:291:22: undefined: bbolt.MmapError

Here, the problem is that Go thinks we want the actual bbolt package, when we actually want our vendored version that adds MmapError. Go modules do not play well with vendoring; the only way to get go build to use our vendored version is to create a go.mod file for Sia with a replace directive:

replace github.com/coreos/bbolt => ./vendor/github.com/coreos/bbolt

Go modules will be enabled by default starting in August (with the release of Go 1.13), so we should make sure we're ready for that. Ideally, we would add a go.mod file right before tagging a new release. This is because, when other modules import Sia, they will automatically select the most recent version (note that the command above select sia@v1.4.0). So we want the automatically-selected version to be compatible with modules.