I've been working with the prelude quite a lot recently. One of the
really painful things with the current implementation is that the
JavaScript for the prelude exists as a const string in a .go file, which
makes properly writing and formatting the JavaScript itself very
difficult.

This PR splits the prelude into separate .js files to make editing the
JavaScript prelude more straightforward.

genmin.go becomes genprelude.go where these .js files are catted
together into a single prelude.go file. The minified prelude_min.go is
as before.

Automated formatting of these .js files will follow in a later PR.

This PR introduces automatic formatting of the prelude's now separate
.js files. Using prettier has the same argument as using gofmt on .go
source code.

As part of this we make the go generate step for the prelude include a
formatpreludejs.go step.

The resulting prelude_min.go file is slightly different now, but post
#791 we can see these
differences are entirely and solely attributable to prettier's adjusting
of the .js files.

Per golang/go#25008 (comment) the
fix for method set calculation will not be back ported to Go 1.10.
Instead gri suggests we vendor the latest version of go/types. This PR
does just that. It is effectively a temporary sticking plaster until Go
1.11 lands, or more specifically until we release GopherJS for Go 1.11.

In order to successfully vendor go/types, however, we need to move
compiler/vendor to the repo root (because build also uses go/types). And
because golang.org/x/tools/go/gcexportdata and
golang.org/x/tools/go/types/typeutil also reference go/types we need to
vendor them as well.

vendor/vendor.json is updated using govendor, with the one exception
being a manual addition for go/types (govendor doesn't appear to
understand how to vendor a standard library package).

Fixes #808.

…#8)

Currently the documentation for js.MakeWrapper is:

> "MakeWrapper creates a JavaScript object which has wrappers for the
exported methods of i. Use explicit getter and setter methods to expose
struct fields to JavaScript."

Where the value a struct value (or more
interestingly a pointer to a struct value) we can actually auto-generate
getters and setters for exported fields in the JavaScript world, rather
than requiring explicit getters and setters to be defined on the Go
side.

We do this via a new MakeFullWrapper method.