julia nightly broke StdWString iteration

[benlorenz]

While trying to adapt libcxxwrap and cxxwrap to julia nightly, in addition to #390 and JuliaInterop/libcxxwrap-julia#137 I got several errors from the new StdString code from #381:

iterate: Error During Test at /home/lorenz/software/polymake/julia/CxxWrap.jl/test/stdlib.jl:130
  Test threw exception
  Expression: iterate(s) == (char, 2)
  MethodError: no method matching between(::UInt32, ::UInt8, ::UInt8)
  
  Closest candidates are:
    between(::T, ::T, ::T) where T<:Integer
     @ Base strings/string.jl:32
  
  Stacktrace:
   [1] _nextind_str
     @ ./strings/string.jl:182 [inlined]
   [2] nextind
     @ ~/software/polymake/julia/CxxWrap.jl/src/StdLib.jl:41 [inlined]
   [3] iterate(s::CxxWrap.StdLib.StdWStringAllocated, i::Int64)
     @ CxxWrap.StdLib ~/software/polymake/julia/CxxWrap.jl/src/StdLib.jl:45
   [4] iterate
     @ ~/software/polymake/julia/CxxWrap.jl/src/StdLib.jl:44 [inlined]
   [5] macro expansion
     @ ~/software/polymake/julia/julia/julia-nightly/share/julia/stdlib/v1.11/Test/src/Test.jl:519 [inlined]
   [6] macro expansion
     @ ~/software/polymake/julia/CxxWrap.jl/test/stdlib.jl:54 [inlined]
   [7] macro expansion
     @ ~/software/polymake/julia/julia/julia-nightly/share/julia/stdlib/v1.11/Test/src/Test.jl:1598 [inlined]
   [8] top-level scope
     @ ~/software/polymake/julia/CxxWrap.jl/test/stdlib.jl:10

The code in question is calling the internal Base._nextind_str:

CxxWrap.jl/src/StdLib.jl

Line 41 in 952a584

Base.nextind(s::CppBasicString, i::Int) = Base._nextind_str(s, i)

But after the recent optimizations in JuliaLang/julia#51671 this stopped working when passing in UInt32 codepoints.

cc: @omus

[barche]

Adding a method to Base.between fixes this (PR #392), not sure if that is a good idea though.