linear MOI model to LPForm conversion

as @frapac suggested, i'll add methods to extract matrices from `EqualTo`, `LessThan` and other constraints

Start by reading the existing MOI-to-LPForm conversion entry points and the handling of EqualTo, LessThan, and other constraints. The issue names no files or tests, so first locate those implementations and their test coverage. Done means matrix extraction methods cover the intended constraint types and the relevant tests pass.

索引モデルが issue の本文から書いたものです。