Draft: coercible signature

[jayzhan211]

Which issue does this PR close?

Closes #.

Rationale for this change

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[jayzhan211]

param_type: target type of function signature
allowed_casts: implicit coercion allowed to cast to target type.

For example,
param_type: string
allowed_casts: binary, int

Valid: All are casted to string
func(string) 
func(binary)
func(int)
Invalid:
func(float or other types)

[alamb]

I am worried that Vec<Vec<ParameterType>> might become challenging to reason about. It would also be confusing about when one use this new signature rather than Signature::Coercable 🤔

Given this seems very similar to Signature::Coercable, and Signature::Coercable mirrors what we want pretty well, could add some new information there on the allowed coercions rather than an entire new type signature. Something like extending Coercable with rules could be used to coerce to the target type:

pub enum TypeSignature {
...
    Coercible(Vec<Coercion>),
...
}

Where Coercion looks like

struct Coercion {
  desired_type: TypeSignatureClass,
  allowed_casts: ... // also includes an option for User Defined?
}

🤔