Make String.aes a stdlib + add more string functions

This means moving the FATE operations to StringInternal and adding to/from_list (and Char.to/from_int
+ Char.to_upper/lower).

test/contracts/bytes_to_x.aes

@@ -1,4 +1,4 @@
-
+include "String.aes"
 contract BytesToX =
 
   entrypoint to_int(b : bytes(42)) : int = Bytes.to_int(b)

test/contracts/funargs.aes

@@ -1,4 +1,4 @@
-
+include "String.aes"
 contract FunctionArguments =
 
   entrypoint sum(n : int, m: int) =

test/contracts/more_strings.aes

@@ -0,0 +1,14 @@
+include "String.aes"
+contract StringX =
+  entrypoint test() =
+    let s1 = "a string"
+    let s2 = "another string"
+    let s  = String.concat(s1, s2)
+    String.sha256(s)
+    String.length(s1)
+    String.from_list(String.to_list(s))
+    String.split(4, s1)
+    String.at(2, s2)
+    String.tokens(s, ",")
+    String.to_upper(s1)
+    String.to_lower(s2)

test/contracts/state_handling.aes

@@ -1,3 +1,4 @@
+include "String.aes"
 contract Remote =
   record rstate = { i : int, s : string, m : map(int, int) }
 

test/contracts/strings.aes

@@ -1,3 +1,4 @@
+include "String.aes"
 contract Strings =
     entrypoint str_len(s) = String.length(s)
     entrypoint str_concat(s1, s2) = String.concat(s1, s2)

test/contracts/unapplied_builtins.aes

@@ -7,6 +7,7 @@
 //    AENS.transfer
 //    AENS.revoke
 //    Oracle.extend
+include "String.aes"
 contract UnappliedBuiltins =
   entrypoint main() = ()
   type o = oracle(int, int)