Avoid needless filters on inferred strategies

[Zac-HD]

#22 notes, among other things, that hypothesis-jsonschema strategies for simple schemas are often slower than the obvious translation. Some of this is unavoidable, as we try some extensive transformations of the schema which are necessary to avoid pathologically poor performance on certain complex schemas, but not all.

There are a range of points at which we output slower strategy which always works, instead of checking whether a simpler one would be correct. For example we always apply a filter to string strategies that validate their length, which is necessary only for regex-based or format-based strings and could be skipped even there in the common case that min_size=0 and max_size=inf.