Misplaced tracebacks from some models created in the reader

[Kodiologist]

$ echo ';;;;;;\na.b' >ex.hy
$ hy ex.hy 
Traceback (most recent call last):
  …
  File "/tmp/ex.hy", line 1, in <module>
    ;;;;;;
NameError: name 'a' is not defined

The error is wrongly attributed to line 1 because the reader fails to set position information for a model created during the expansion of a.b into (. a b). Contrast with this case, which is correct:

$ echo ';;;;;;\n(. a b)' >ex.hy
$ hy ex.hy                     
Traceback (most recent call last):
  …
  File "/tmp/ex.hy", line 2, in <module>
    (. a b)
NameError: name 'a' is not defined

I believe it suffices to call model.replace(model) in HyReader.fill_pos, so that positions are fixed recursively. I just need to write a test.

This bug is more consequential than it sounds because it can crash pytest internally, when non-existent columns of the source are retrieved for a traceback. Hy's own error reporting doesn't fail so badly in such a case.