Add string functions about rchip-proposals HOT 3 OPEN

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I struggled to concatenate a number and a string ultimately discovering it is impossible in rholag.

Are you quite sure about that, @jimscarver ? String interpolation works when I try it in the rholang playground:

new x in {
  x!("${n}${s}" %% {"n": 123, "s": "abc"})
}

Result

[
  "123abc"
]

from rchip-proposals.

There are many other string functions that are needed like case conversion, finding a substring, and matching patterns.

Regular expressions prove to be problematical due to recursion see #19 however filename "glob" pattern matching like [Ss]ubstring are not recursive and could be supported easily https://index.scala-lang.org/salva/scala-glob/scala-glob/0.0.3?

We can also support more types for existing functions easily like string.contains(substring). Any type that makes sense in a function should be supported. This could be done even by scala newbees

from rchip-proposals.

We also need a simple String.match capability being considered in #14

from rchip-proposals.