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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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