Make formulae reach transformations before outer names

[tomicapretto]

All the environment machinery built into formulae was working in such a way that names in "outer" environments were reached before the names of "internal" transformations. This caused these kind of problems

p = 1
design_matrices("p(y, n) ~ 1", df)

TypeError: 'int' object is not callable

This PR changes the order in which namespaces are added into the environment so transformations and encodings are reached first.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9bcd01a) 92.62% compared to head (7df836c) 92.63%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #109   +/-   ##
=======================================
  Coverage   92.62%   92.63%           
=======================================
  Files          20       20           
  Lines        2225     2227    +2     
=======================================
+ Hits         2061     2063    +2     
  Misses        164      164           

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