consider length of string keys

[mdrossmiller]

take length of keys in the str_set hash into account so keys which are perfect sub-strings do not match incorrectly

[mikkelfj]

thanks

[mikkelfj]

Note: this is a low impact issue unless str_set is used separately from flatcc as is. See also CHANGELOG note. Essentially str_set is used to detect if a header file has already been included in generated output. A false positive of a prefix match is unlikely due to hashing, and if it happens, worst case the end code might not compile without manual inclusion of a file assumed to be already included.