core: replace manual uniplate impls with derive macro

Replace the manual implementation of the Uniplate and Biplate traits for
the AST with the derive macro.

Adding uniplate_derive to the ast broke is_enum_variant, therefore this
commit also removes and replaces all invocations of is_xxx() functions
with let..else or pattern matching.

Signed-off-by: Niklas Dewally <niklas@dewally.com>