Fix nested ordering in Formula instances, and reduce redundant operations in Structured preparation.

[matthewwardrop]

from formulaic import Formula

{
"degree": Formula("z + z:a + z:b:a + g"),
"none": Formula("z + z:a + z:b:a + g", _ordering="none"),
"sort": Formula("z + z:a + z:b:a + g", _ordering="sort"),
}

{
"degree": Formula("c ~ z + z:a + z:b:a + g").rhs,
"none": Formula("c ~ z + z:a + z:b:a + g", _ordering="none").rhs,
"sort": Formula("c ~ z + z:a + z:b:a + g", _ordering="sort").rhs,
}

Both now output:

{
"degree": Formula("c ~ z + z:a + z:b:a + g").rhs,
"none": Formula("c ~ z + z:a + z:b:a + g", _ordering="none").rhs,
"sort": Formula("c ~ z + z:a + z:b:a + g", _ordering="sort").rhs,
}

Formerly, the terms would always be sorted at least by degree in the latter case.

closes: #191