Add AsStr trait for convert_error

[dalance]

convert_error function takes &str only. So if I use an input type except &str like nom_locate::LocatedSpanEx<&str, ()>, convert_error can't be used.

This PR add AsStr trait to provide as_str().
Hereby any input type implementing AsStr can be used at convert_error.

[coveralls]

Coverage increased (+1.2%) to 87.489% when pulling f10d1c8 on dalance:as_str_trait into 8517064 on Geal:master.

[Geal]

hello, I'll close this in favor of b2ed755 which uses a more standard trait. Thanks for the push anyway!