Introduce declarations to haskell-yas.el so it byte-compiles cleanly.

Without them:

```
In haskell-snippets-initialize:
haskell-yas.el:60:17:Warning: reference to free variable `yas-snippet-dirs'
haskell-yas.el:61:23:Warning: assignment to free variable `yas-snippet-dirs'

In end of data:
haskell-yas.el:73:1:Warning: the function `yas-load-directory' is not known to
    be defined.
```