[michaelklishin]

This implements the design in #69 with a few deviations or additions:

1. alias is a single string key property accepted as a common alternative to aliases, an array of strings
2. This PR adds a few property-based test suites
3. Conflicting aliases are considered to be schema integrity errors
4. Conflicting canonical keys and aliases are resolved like so: the canonical (original) value wins
5. Conflicting values on alias lists are resolved in an equally straightforward manner: the first value on the list wins
6. {aliases, []} in a merge mapping is a deliberate "clear all aliases" directive - it clears aliases inherited from the base mapping. This is intentionally asymmetric with see and doc, where an empty list in a merge mapping means "not specified, inherit".

RHS Substitution Limitation

Alias keys are rewritten to their canonical keys early in the pipeline, before RHS substitutions are evaluated. This means $(old.key) in a substitution will fail if old.key is an alias - use $(new.key) instead. Cuttlefish will emit a specific error message identifying the alias and naming the canonical key to use.

A Realistic Example

In this hypothetical (for now anyway) scenario, assume that Team RabbitMQ
wants to move a few rabbitmq.conf keys under the raft.quorum_queue.* namespace (raft.{subsystem}.{key}, if we generalize).

Today this requires duplicating the mapping and writing boilerplate translations
for every key, and making sure you get it right every time:

{mapping, "raft.quorum_queue.segment_max_entries", "ra.segment_max_entries", [
    {datatype, integer},
    {validators, ["non_zero_positive_integer", "positive_16_bit_unsigned_integer"]}
]}.
{mapping, "raft.segment_max_entries", "ra.segment_max_entries", [
    {datatype, integer}
]}.
{translation, "ra.segment_max_entries", fun(Conf) ->
    case cuttlefish:conf_get("raft.quorum_queue.segment_max_entries", Conf, undefined) of
        undefined -> cuttlefish:conf_get("raft.segment_max_entries", Conf, cuttlefish:unset());
        Value     -> Value
    end
end}.

With aliases, the old keys just work and the implementation is dramatically less error-prone:

{mapping, "raft.quorum_queue.segment_max_entries", "ra.segment_max_entries", [
    {datatype, integer},
    {validators, ["non_zero_positive_integer", "positive_16_bit_unsigned_integer"]},
    %% a shortcut for {aliases, ["raft.wal_max_size_bytes"]}
    {alias, "raft.wal_max_size_bytes"}
]}.

A Basic Conflict Resolution Example

If both the canonical key and the alias are set, the canonical key takes precedence:

# rabbitmq.conf
raft.quorum_queue.wal_max_size_bytes = 536870912
raft.wal_max_size_bytes = 268435456

{mapping, "raft.quorum_queue.wal_max_size_bytes", "ra.wal_max_size_bytes", [
    {datatype, integer},
    {validators, ["non_zero_positive_integer"]},
    {alias, "raft.wal_max_size_bytes"}
]}.

The outcome: the value of 536870912 will be used, and Cuttlefish will emit a warning:

raft.wal_max_size_bytes is deprecated and ignored in favor of raft.quorum_queue.wal_max_size_bytes

Other Comments

Closes #69.

[lukebakken]

Feel free to ship 3.7 if you'd like, thanks!