Add an attr.Convert helper.

[paddycarver]

This function converts an attr.Value to use a new attr.Type, so long
as they share a compatible tftypes.Type representation. This allows
code that doesn't know the type of an attr.Value to still use it,
provided they know the kind of data it holds.

For example, a validation helper that works on strings--MaxStringLength,
say--could look something like this:

// assume req.Config is an attr.Value
strAttr, err := attr.Convert(ctx, req.Config, types.StringType)
if err != nil {
  panic(err)
}
var str types.String
diags := tfsdk.ValueAs(ctx, strAttr, &str)
if diags.HasError() {
  panic("oops")
}
// do something with `str`, which is now strongly typed

Then no matter what attr.Type provider devs use in their schema, this
validator would still work, then. Without the conversion step, it
wouldn't work, as tfsdk.ValueAs implementation needs to know the
attr.Type of the attr.Value, and reusable validators can't know
that. So the point of this helper is to get an attr.Value into a known
attr.Type format.

This still needs tests.

[paddycarver]

Depends on #119.

[paddycarver]

Still depends on #119, but has tests and is otherwise code complete.

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