add Dict fuzzers: dict, dict_between, dict_at_least, dict_at_most, dict_from

[MavenRain]

Closes #24 (the aiken/fuzz/dict bullets from #2).

Adds Dict fuzzers to aiken/fuzz, next to the set family:

• dict(fuzz_key, fuzz_value): up to 20 elements
• dict_between(fuzz_key, fuzz_value, min, max)
• dict_at_least(fuzz_key, fuzz_value, min)
• dict_at_most(fuzz_key, fuzz_value, max)
• dict_from(fuzz_keys, fuzz_value): one value per sampled key

Design notes:

• Keys come from set_between, so they are unique and the size distribution is the documented set_between one. Values are generated one per key and inserted with dict.insert.
• The key fuzzer is Fuzzer<ByteArray> rather than the Fuzzer<k> written in TODOs #2: Dict stores keys as ByteArray internally and dict.insert takes key: ByteArray. The phantom key type parameter is left free for the caller to pin.
• dict_from collapses duplicate keys (last generated value wins), so the result can be smaller than the sample.
• Placement follows Added rational number fuzzer functions and corresponding tests #19 (rational fuzzers): directly in aiken/fuzz, not a submodule, which also avoids a name clash between a fuzz/dict module and the stdlib dict module.

Testing: 7 new property tests in lib/aiken/fuzz.test.ak covering size bounds for all four size variants, key uniqueness and strict key ordering, value ranges, and duplicate collapse in dict_from. Full suite: 48/48 green under the pinned compiler (v1.1.17); formatter check (fmt --check) clean under v1.1.17.