ommx::random submodule, proptest::Arbitrary-based random object generation

[termoshtt]

For benchmark input in #329

Changes

• Create structs represents the input of Arbitrary::arbitrary_with, e.g. LinearParameters and InstanceParameters
• Move impl Arbitrary to src/random/*.rs from src/convert/*.rs
• Make arbitrary_xxx helper functions public under ommx::random::
• New random and random_deterministic functions to create e.g. Linear from LinearParameters