Refactor import-loading to simplify the type-generation code (#101)

## Summary:
genqlient has some code (`imports.go`) dedicated to tracking which
imports we need and avoiding conflicts, as well as converting a
(restricted) Go expression like `map[string]github.com/me/mypkg.MyType`
to an import (`github.com/me/mypkg`) and a type-reference
(`map[string]mypkg.MyType`) to be used in the context of that import,
and at least making some attempt to track conflicts.  (Right now the
conflict-avoidance is not very smart, and not very well tested, but it
comes up rarely anyway.)  Sadly, that code was a bit cumbersome to use,
because you had to first register the imports (typically from
`convert.go`), then use them (often from the template).

In this commit I refactor the order we write things in order to allow a
significant simplification of how we import; in particular we no longer
have to guess in advance what imports which template will need; it can
just do `{{ref <expr>}}` as before, and it just works.  To do this, I:
- changed the importer to have only one API, which adds the import if
  needed, and returns the reference either way
- added a check that we don't add imports after they're written
- reorganized the toplevel templates a bit to make sure that check never
  fires; we now generate all the types and operations, then write the
  imports and glue it all together
This removes a bunch of silly code, and should simplify the process of
adding custom (un)marshalers (#38).

While I was at it, I put the documentation of what expressions we
support in a more visible place, and added a type-assertion that your
custom context type implements context.Context (if applicable).

## Test plan:
make check


Author: benjaminjkraft

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

Required Reviewers: 

Approved By: StevenACoffman, dnerdy

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/101

docs/genqlient.yaml

@@ -93,6 +93,16 @@ bindings:
     #  time.Time
     #  map[string]interface{}
     #  github.com/you/yourpkg/subpkg.MyType
+    # Specifically, this can be any of the following expressions:
+    # - any named type (qualified by the full package path)
+    # - any predeclared basic type (string, int, etc.)
+    # - interface{}
+    # - for any allowed type T, *T, []T, [N]T, and map[string]T
+    # but can't be, for example:
+    # - an inline (unnamed) struct or interface type
+    # - a map whose key-type is not string
+    # - a nonstandard way of spelling those, (interface {/* hi */},
+    #   map[  string      ]T)
     type: time.Time
 
   # To bind an object type: