Brian Goff 1ad87c6ba6 Update go-cty to pull in more stdlib funcs.
I needed "split" specifically so I can do something like:

```hcl
variable PLATFORMS {
  default = "linux/amd64"
}

target foo {
  platforms = split(",", "${PLATFORMS}")
  # other stuff
}
```

Where the existing "csvdecode" does not work for this because it parses
the string into a list of objects instead of a list of strings.

I went ahead and just added all the available new functions.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>
..
stdlib Update go-cty to pull in more stdlib funcs.
argument.go Upgrade hcl to v2
doc.go Upgrade hcl to v2
error.go Upgrade hcl to v2
function.go Upgrade hcl to v2
unpredictable.go Upgrade hcl to v2