Performance issue in Options.namer property

[benc64]

The Options.namer property currently instantiates a StackFrameNamer via get_default_namer() even when the namer has been set by calling options.with_namer(n) and the created StackFrameNamer instance will never be used.
In my situation, this caused a noticeable performance hit that showed up under the profiler.

Please consider only calling get_default_namer() if the namer field has not been set.

Current state:

    @property
    def namer(self) -> Namer:
        from approvaltests.namer.default_name import get_default_namer

        namer = self.fields.get("namer", get_default_namer())
        if hasattr(namer, "set_extension"):
            namer.set_extension(self.for_file.file_extention)
        return namer

Potential fix:

    @property
    def namer(self) -> Namer:
        from approvaltests.namer.default_name import get_default_namer

        namer = options.fields.get("namer", None)
        if namer is None:
            namer = get_default_namer()

        if hasattr(namer, "set_extension"):
            namer.set_extension(self.for_file.file_extention)
        return namer

Workaround:
passing the namer explicitly to calls to verify bypasses the use of the Options.namer property

PS. I love ApprovalTests!