Unicode compatibility: replace index arithmetics by prevind

[hhaensel]

There are two cases where unicode characters in f"..." expressions currently fail due to index arithmetics:

• directly preceeding expressions
• at the end of the expression

Replacing j-1 by prevind(s, j) solves the problem.

I have included two test cases in the runtest.jl which currently fail:

@test f"π\(2*2)" == "π4"
@test f"π = \(2*90)°" == "π = 180°"

[ScottPJones]

Thanks! I'm too used to my Str strings, that usually don't need nextind or prevind

[hhaensel]

Welcome! Just came across one situation where it failed...
... and through your comment I learned how to use Strs.jl :-)