fix parameter confusion in macros

[divarvel]

macros resolve scope parameters and regular parameters through the ToAnyParams trait, losing the ability to discriminate between the two in a static way. This leads to panics instead of compilation errors.

Note: this PR fixes the issue of mistakenly providing a term-like value (most likely a string) where a public key is expected. It also removes the provided impl ToAnyParam for PublicKey. Since this implementation never makes sense with the other changes, it’s fine to remove it as a bugfix.
The code still uses ToAnyParam for datalog terms, which allows to provide a public key value when implementing it.
Completely removing this possibility requires removing the ToAnyParam conversion trait altogether, which requires careful thought. This is what #314 is about.

That being said, this PR already removes the possibility of honest mistakes when using macros, so it’s valuable in itself.

Fixes #301

ToDo

• store separately parameters and scope parameters in Item
• only use ToAnyParam for datalog terms
• require PublicKey directly for scope params
• remove impl ToAnyParam for PublicKey
• try using Into<Term> instead of T: ToAnyParam
• try using Into<(PublicKey, Vec<PublicKey>)> instead of PublicKey