moved repr from io.jl to string.jl

...as it is used in pcre.jl

Before:
````
julia> Regex(":)")
ERROR: repr not defined
 in compile at pcre.jl:85
 in Regex at regex.jl:18
 in Regex at regex.jl:34
````

After:
````
julia> Regex(":)")
ERROR: compile: unmatched parentheses at position 2 in ":)"
 in compile at pcre.jl:85
 in Regex at regex.jl:18
 in Regex at regex.jl:34
````