Update type signature for `reflectSymbol` for PureScript 0.15

[thomashoneyman]

As discussed in #230, we need to change this type signature:

purescript-prelude/src/Data/Symbol.purs

Line 23 in 1100459

reflectSymbol :: forall proxy. proxy sym -> String

...when PureScript 0.15 comes out. The signature is using a type variable proxy instead of Proxy just to avoid too much breakage in 0.14, but we'll want to update it in the next compiler version.