add halicore_symbol

[dagit]

Add an isabelle command like this: halicore_symbol "<$>" "zlzdzg"

The first parameter is the name as it appears in the Haskell program and the second parameter is the matching z encoding.

The idea is that this would allow halicore syntax snippets to support the Haskell name. The reason for the halicore_syntax command is that it needs to notify the Isabelle parser.