Add a mechanism to specify options on input-type fields (#124)

## Summary:
This has been a bit of a thorn since we started using genqlient in
production: just as you might want to specify, say, `omitempty` on an
argument, you might equally want to specify it on an input-type field.
But there's no obvious syntax to do that, because the input-type field
does not appear in the query (only the schema) so there's nowhere to put
the `# @genqlient` directive.

This commit, at last, fixes that problem, via a new option, `for`, which
you use in an option applied to the entire operation (or fragment), and
says, "actually, apply this directive to the given field, not the entire
operation".  (It's mainly useful for input types, but I allowed it for
output types too; I could imagine it being convenient if you want to say
you always use a certain type or type-name for a certain field.)  It
works basically like you expect: the inline options take precedence over
`for` take precedence over query-global options.

The implementation was fairly straightforward once I did a little
refactoring, mostly in the directive-parsing and directive-merging
(which are now combined, since merging is now a bit more complicated).
With that in place, and extended to support `for`, we need only add the
same wiring to input-fields that we have for other places you can put
directives.  I did not attempt to solve the issue I've now documented
as #123, wherein conflicting options can lead to confusing behavior;
the new `for` is a new and perhaps more attractive avenue to cause it
but the issue remains the same and requires nontrivial refactoring
(described in the issue) to solve.  (The breakage isn't horrible for the
most part; the option will just apply, or not apply, where you don't
expect it to.)

But while applying that logic, I noticed a problem, which is that we
were inconsistently cascading operation-level options down to
input-object fields.  (I think this came out of the fact that initially
I thought to cascade them, then realized that this could cause problems
like #123 and intended to walk them back, but then accidentally only
"fixed" it for `omitempty`.  I guess until this change, operation-level
options were rare enough, and input-field options messy enough, that no
one noticed.)  So in this commit I bring things back into consistency,
by saying that they do cascade: with at least a sketch of a path forward
to solve #123 via better validation, I think that's by far the clearest
behavior.

Issue: https://github.com/Khan/genqlient/issues/14

## Test plan:
make check


Author: benjaminjkraft

Reviewers: csilvers, StevenACoffman, benjaminjkraft, aberkan, dnerdy, jvoll, mahtabsabet, MiguelCastillo

Required Reviewers: 

Approved By: csilvers, StevenACoffman

Checks: ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/124

docs/CHANGELOG.md

@@ -22,10 +22,13 @@ When releasing a new version:
 
 ### Breaking changes:
 
+- Previously, `# @genqlient` directives applied to entire operations applied inconsistently to fields of input types used by those operations.  Specifically, `pointer: true`, when applied to the operation, would affect all input-field arguments, but `omitempty: true` would not.  Now, all options apply to fields of input types; this is a behavior change in the case of `omitempty`.
+
 ### New features:
 
 - genqlient's types are now safe to JSON-marshal, which can be useful for putting them in a cache, for example.  See the [docs](FAQ.md#-let-me-json-marshal-my-response-objects) for details.
 - The new `flatten` option in the `# @genqlient` directive allows for a simpler form of type-sharing using fragment spreads.  See the [docs](FAQ.md#-shared-types-between-different-parts-of-the-query) for details.
+- The new `for` option in the `# @genqlient` directive allows applying options to a particular field anywhere it appears in the query.  This is especially useful for fields of input types, for which there is otherwise no way to specify options; see the [documentation on handling nullable fields](FAQ.md#-nullable-fields) for an example, and the [`# @genqlient` directive reference](genqlient_directive.graphql) for the full details.
 
 ### Bug fixes: