auto: rm std::map::StdMap trait, rename std::map to std::oldmap and start transition to the core::container::Map API

The rationale for renaming is that there isn't any reason to prefer `std::map` (not sendable, legacy API, and slower) over `core::hashmap::linear`, and it makes it obvious that the module in core is the one to use. Migrating to `core::hashmap` will take a fair bit of a work, but in most cases it should be as simple as marking the variable as mutable and replacing a few copies with borrowed pointers.

Any future hash table implementations will implement the `core::container::Map` trait so migrating between them will be simple. My plan is to incrementally turn the old map into an owned `core::hashmap::chained` container. It may or not be a useful container worth keeping around, but it's too hard to migrate right to `LinearMap` everywhere at once.

Here are the current benchmarks:

    % ./core-map 1000
    std::map::HashMap
                   sequential_ints 0.001247 s
                       random_ints 0.001063 s
                       delete_ints 0.000392 s
                sequential_strings 0.002818 s
                    random_strings 0.00088 s
                    delete_strings 0.000348 s
    core::hashmap::linear::LinearMap
                   sequential_ints 0.000416 s
                       random_ints 0.000365 s
                       delete_ints 0.000139 s
                sequential_strings 0.000822 s
                    random_strings 0.00059 s
                    delete_strings 0.000329 s

    % ./core-map 10000
    std::map::HashMap
                   sequential_ints 0.009247 s
                       random_ints 0.00382 s
                       delete_ints 0.001583 s
                sequential_strings 0.014042 s
                    random_strings 0.008566 s
                    delete_strings 0.00445 s
    core::hashmap::linear::LinearMap
                   sequential_ints 0.004112 s
                       random_ints 0.003433 s
                       delete_ints 0.001645 s
                sequential_strings 0.008921 s
                    random_strings 0.006459 s
                    delete_strings 0.004639 s

    % ./core-map 100000
    std::map::HashMap
                   sequential_ints 0.092820 s
                       random_ints 0.076495 s
                       delete_ints 0.027946 s
                sequential_strings 0.171406 s
                    random_strings 0.118372 s
                    delete_strings 0.055188 s
    core::hashmap::linear::LinearMap
                   sequential_ints 0.062137 s
                       random_ints 0.050849 s
                       delete_ints 0.018603 s
                sequential_strings 0.119988 s
                    random_strings 0.071857 s
                    delete_strings 0.047536 s

    % ./core-map 1000000
    std::map::HashMap
                   sequential_ints 0.877039 s
                       random_ints 0.890236 s
                       delete_ints 0.278055 s
                sequential_strings 1.756422 s
                    random_strings 1.576072 s
                    delete_strings 0.595255 s
    core::hashmap::linear::LinearMap
                   sequential_ints 0.643638 s
                       random_ints 0.503949 s
                       delete_ints 0.226117 s
                sequential_strings 1.185846 s
                    random_strings 0.739722 s
                    delete_strings 0.528093 s