Unfuse `temp_param`

[pgiarrusso-sl]

@gmalecha mentioned in #274 (comment) that

Unfusing temp_param would simplify some additional code.

I'm unsure how temp_param is fused, but recording an issue for later investigation.

[gmalecha]

Effectively, temp_param ~ ident * template_kind. The two actions here would be either:

1. Add the default value, which would probably be a bit annoying because it grows the mutual inductive.
2. Explicitly introduce template_kind and replace this with a pair.

Generally, BRiCk avoids pairs in the AST because they are not efficient to represent/type check.