This PR moves the constructor layout code from C++ to Lean. When
writing the new compiler, we just reused the existing C++ code,
even though it was a bit inconvenient, because we wanted to
ensure that constructor layout always matched the existing
compiler.
This fixes#2589 by handling struct field types just like any
other type being lowered, and thus applying the trivial structure
optimization in the process. Originally, I wanted to port the
code to Lean without any functional changes, but I found that
it took less code to just implement it "correctly" and get this
fix as a consequence than to emulate the bugs of the existing
C++ implementation.
