Nat generics

[utkn]

This PR aims to extend the generics to support numerics. Most of the changes are in Syntax.lean.

At Lean side, generics can be declared as numeric with the # prefix, as shown in the following example:

nr_def create_arr<#N>() -> [Field; N] {
  [1 : Field ; N]
}

[mpenciak]

Looks good.

Good catch on str<N> and fmtstr<N,...>, I overlooked that strings could take const generics as length parameters.