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/function/stdlib/conversion.go

@@ -0,0 +1,87 @@
+package stdlib
+
+import (
+	"strconv"
+
+	"github.com/zclconf/go-cty/cty"
+	"github.com/zclconf/go-cty/cty/convert"
+	"github.com/zclconf/go-cty/cty/function"
+)
+
+// MakeToFunc constructs a "to..." function, like "tostring", which converts
+// its argument to a specific type or type kind.
+//
+// The given type wantTy can be any type constraint that cty's "convert" package
+// would accept. In particular, this means that you can pass
+// cty.List(cty.DynamicPseudoType) to mean "list of any single type", which
+// will then cause cty to attempt to unify all of the element types when given
+// a tuple.
+func MakeToFunc(wantTy cty.Type) function.Function {
+	return function.New(&function.Spec{
+		Params: []function.Parameter{
+			{
+				Name: "v",
+				// We use DynamicPseudoType rather than wantTy here so that
+				// all values will pass through the function API verbatim and
+				// we can handle the conversion logic within the Type and
+				// Impl functions. This allows us to customize the error
+				// messages to be more appropriate for an explicit type
+				// conversion, whereas the cty function system produces
+				// messages aimed at _implicit_ type conversions.
+				Type:      cty.DynamicPseudoType,
+				AllowNull: true,
+			},
+		},
+		Type: func(args []cty.Value) (cty.Type, error) {
+			gotTy := args[0].Type()
+			if gotTy.Equals(wantTy) {
+				return wantTy, nil
+			}
+			conv := convert.GetConversionUnsafe(args[0].Type(), wantTy)
+			if conv == nil {
+				// We'll use some specialized errors for some trickier cases,
+				// but most we can handle in a simple way.
+				switch {
+				case gotTy.IsTupleType() && wantTy.IsTupleType():
+					return cty.NilType, function.NewArgErrorf(0, "incompatible tuple type for conversion: %s", convert.MismatchMessage(gotTy, wantTy))
+				case gotTy.IsObjectType() && wantTy.IsObjectType():
+					return cty.NilType, function.NewArgErrorf(0, "incompatible object type for conversion: %s", convert.MismatchMessage(gotTy, wantTy))
+				default:
+					return cty.NilType, function.NewArgErrorf(0, "cannot convert %s to %s", gotTy.FriendlyName(), wantTy.FriendlyNameForConstraint())
+				}
+			}
+			// If a conversion is available then everything is fine.
+			return wantTy, nil
+		},
+		Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+			// We didn't set "AllowUnknown" on our argument, so it is guaranteed
+			// to be known here but may still be null.
+			ret, err := convert.Convert(args[0], retType)
+			if err != nil {
+				// Because we used GetConversionUnsafe above, conversion can
+				// still potentially fail in here. For example, if the user
+				// asks to convert the string "a" to bool then we'll
+				// optimistically permit it during type checking but fail here
+				// once we note that the value isn't either "true" or "false".
+				gotTy := args[0].Type()
+				switch {
+				case gotTy == cty.String && wantTy == cty.Bool:
+					what := "string"
+					if !args[0].IsNull() {
+						what = strconv.Quote(args[0].AsString())
+					}
+					return cty.NilVal, function.NewArgErrorf(0, `cannot convert %s to bool; only the strings "true" or "false" are allowed`, what)
+				case gotTy == cty.String && wantTy == cty.Number:
+					what := "string"
+					if !args[0].IsNull() {
+						what = strconv.Quote(args[0].AsString())
+					}
+					return cty.NilVal, function.NewArgErrorf(0, `cannot convert %s to number; given string must be a decimal representation of a number`, what)
+				default:
+					return cty.NilVal, function.NewArgErrorf(0, "cannot convert %s to %s", gotTy.FriendlyName(), wantTy.FriendlyNameForConstraint())
+				}
+			}
+			return ret, nil
+		},
+	})
+}