Add `string` type

[DanGooding]

This should allow a bunch more general purpose programming. Important features are

• reading input lines
• parsing of numbers
• formatting of numbers/bools/tuples/lists
  • int-to-string functions
  • format strings: compile-time syntactic sugar for join(["before", int-to-string(d), "after"])
• unicode support? - this makes most character-wise operations more complex
• join/split/substring
• regex - query, maybe match

implementation concerns:

• store the length, rather than being null terminated
• implement a bunch of operations in the runtime