Store response objects in a Vec with ArcStr names - #1416

Each resolved field allocated a `String` for its response name. The
asynchronous executor allocated the name twice. `Object` also hashed
each name with SipHash up to three times per field. It hashed the name
to find an existing field and to insert the new one.

`Object` stores its fields in a `Vec` in insertion order. A lookup
compares the names one by one. For the small field counts of GraphQL
objects, this is faster than hashing. Above 16 fields, `Object` builds
a hash index of the names, so a selection set with thousands of aliases
cannot make execution quadratic. A `Box` keeps the index out of
`Value`, so `Value` stays at 32 bytes. The field names are `ArcStr`. A
field without an alias reuses the `ArcStr` name from the schema, so its
response name needs no allocation. The logic that merges fields with
the same response name moved from the executor into `Object`.

A synchronous execution of 1000 objects with 6 fields each now makes
4061 allocations instead of 11,063. On the benchmark operations,
synchronous execution takes 4 to 44% less time, and asynchronous
execution takes 4 to 39% less time.

New tests cover adding, merging, and comparing object fields, objects
with an index, and the insertion order.