[WIP] Shumo add string functions

Review after #164.

String functions:
* [ ] add `substr(str, begin, end)` function, which will give subtring of `str` from  index `begin` (inclusive) to index `end` (exclusive)
* [ ] add 'tail(str, length)' function, which will return substring of `str` from its head with length `length`. If `len(str) < length`, it will return a copy of itself.
* [ ] add 'head(str, length) ' function, which will return substring of `str` at its end with length `length`. If `len(str) < length`, it will return a copy of itself.

Changes on RACO:
* [ ] implement NaryFunction.