fix stack overflow error and reduce public API (rebased #61)

[lovasoa]

I merged master into #61 and fixed the conflicts

• remove extra whitespace in expected &strs in test_quotations()
• refactor split_constant_and_expr() to being an implementation of LpExpression, working on &self
• refactored to have an LpExprArena, still needs reorganizing around it
• mostly compiling code, only macros in dsl/operations.rs are non-functional, as they would have to take expr_arena as an extra argument that basic std::ops like Sub cannot
• remove operations.rs submodule
• version of Add, Sub, Mul implementations, just not working...
• Revert "remove operations.rs submodule"
• compiling version with Add, Sub and Mul implemented for LpExprArena
• make sum and lp_sum work on Into instead of Into
• most bugs should be fixed by testing, now
• all warnings removed, extra match case in split_off_constant
• tests all passing, code should be debugged -- yay!
• refactor some LpExprArena functions for consistency and comprehension
• refactor LpExpression to LpExprNode and LpExprArena to LpExpression; adapt native_cbc.rs to new architecture
• debug compilation with --features "native_coin_cbc"
• remove println()s used for debugging
• restrict most LpExpression functions to pub(crate), move respective tests from tests/expression.rs and tests/quote.rs to test mod in variables.rs
• remove unused if let statement
• mark LpExpression::build() as only being used in tests
• remove unused function LpExprNode::normalize()
• make LpExpression::new() private to the module
• make LpExpression::set_root_index() private to module
• make LpExpression::push_as_expr() private to module variables.rs
• make LpExpression::clone_expr_at_and_push() private to module variables.rs
• make LpExpression::overwrite_expr_at() private to module variables.rs
• make LpExpression::expr_clone_at() private to module variables.rs
• remove LpExpression::get_root_expr() that was only used once and was a one-liner
• make LpExpression::push_arena_at_root() private to module variables.rs
• make LpExpression::clone_subtree_at_index_and_push() private to module variables.rs
• move LpExpression::var_lit() back to native_cbc.rs, as that's the only place it is used
• Add support for minilp
• Add missing feature gate
• Run tests with the minilp feature
• Update README.md

[lovasoa]

I tried to add a test to see if it fixes #37, but it looks like it doesn't :

    #[test]
    fn simplify_deep_sum() {
        let count = 1000;
        let vars: Vec<LpExpression> = (0..count)
            .map(|i|
                &LpInteger::new(&format!("v{}", i)) * 2 + 1
            )
            .collect();

        let mut sum = lp_sum(&vars);
        let c = sum.simplify().split_off_constant();
        assert_eq!(c, count as f32);
    }

So it looks like it signicantly increases complexity, without fixing the initial issue :/

[lovasoa]

It looks like simplify repetitively calls show, which is a recursive function that allocates a large string !

[lovasoa]

@dlaehnemann @jcavat : I removed the recursivity in Show. It is still super slow (allocating a ton of strings during simplify is not a good idea), but at least it works now, it does not throw a stack error.

[jcavat]

Great job guys! I merge and wait few days before upgrading to version 0.4.4 (or 0.5.0). (I will be off next week)