add base2string and string2base to string.k

@andreistefanescu please review.

Fixes #573
```
dwightguth@rvwork-1:~/k$ grep -i string2base -R .
./lib/maude/lib/builtins.maude:  op string2Base : #String #NzNat -> #Int .
./lib/maude/lib/builtins.maude:ceq string2Base(S:#String, N:#NzNat) =
I:#Int if I:#Int := String2Rat(S:#String, N:#NzNat) .
./lib/maude/MaudeHooksMap.properties:\#CONVERSION\:string2Base=\#CONVERSION\:string2Base
./include/java/hooks.properties:\#CONVERSION\:string2Base :
org.kframework.backend.java.builtins.BuiltinStringOperations.string2base
./.git/logs/HEAD:7e26ab2763545262b71c920119a606bbee48de13
ed6e8ddc680019750ccd75c3c7711a1a01cd7c4e Dwight Guth <
dwightguth@runtimeverification.com> 1403193984 -0500    commit: add
base2string and string2base to string.k
./.git/logs/refs/heads/base2string:7e26ab2763545262b71c920119a606bbee48de13
ed6e8ddc680019750ccd75c3c7711a1a01cd7c4e Dwight Guth <
dwightguth@runtimeverification.com> 1403193984 -0500    commit: add
base2string and string2base to string.k
./.git/COMMIT_EDITMSG:add base2string and string2base to string.k
./src/javasources/KTool/src/org/kframework/backend/java/builtins/BuiltinStringOperations.java:
public static IntToken string2base(StringToken term, IntToken base,
TermContext context) {
```

Looks like the hook was made but not the K operation in the prelude.


On Thu, Jun 19, 2014 at 11:18 AM, Andrei Stefanescu <
notifications@github.com> wrote:

> In include/builtins/string.k:
>
> > @@ -42,6 +42,8 @@ module STRING-HOOKS
> >    syntax Float ::= "String2Float" "(" String ")" [function, klabel(String2Float), hook(#CONVERSION:string2float)]
> >    syntax Int ::= "String2Int" "(" String ")"	 [function, klabel(String2Int), hook(#CONVERSION:string2int)]
> >    syntax String ::= "Int2String" "(" Int ")"	 [function, klabel(Int2String), hook(#CONVERSION:int2string)]
> > +  syntax String ::= "Base2String" "(" Int "," Int ")" [function, klabel(Base2String), hook(#CONVERSION:base2string)]
> > +  syntax String ::= "String2Base" "(" String "," Int ")" [function, klabel(String2Base), hook(#CONVERSION:string2base)]
>
> I thought there was already a string2base which was hooked.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/kframework/k/pull/631/files#r13977527>.
>