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>

vendor/github.com/zclconf/go-cty/cty/path.go

@@ -51,11 +51,31 @@ func (p Path) Index(v Value) Path {
 	return ret
 }
 
+// IndexInt is a typed convenience method for Index.
+func (p Path) IndexInt(v int) Path {
+	return p.Index(NumberIntVal(int64(v)))
+}
+
+// IndexString is a typed convenience method for Index.
+func (p Path) IndexString(v string) Path {
+	return p.Index(StringVal(v))
+}
+
 // IndexPath is a convenience method to start a new Path with an IndexStep.
 func IndexPath(v Value) Path {
 	return Path{}.Index(v)
 }
 
+// IndexIntPath is a typed convenience method for IndexPath.
+func IndexIntPath(v int) Path {
+	return IndexPath(NumberIntVal(int64(v)))
+}
+
+// IndexStringPath is a typed convenience method for IndexPath.
+func IndexStringPath(v string) Path {
+	return IndexPath(StringVal(v))
+}
+
 // GetAttr returns a new Path that is the reciever with a GetAttrStep appended
 // to the end.
 //