- Instead of adding a separate module `schema` like there was before,
since service parsing and executable parsing have a fair amount of
overlap I put them as two submodules `executable` and `service` in both
`parse` and `types`. Also, the grammar is unified under one `.pest`
file.
- Added const equivalents to `Value`, `Directive` etc
- Change the reexport `async_graphql::Value` from
`async_graphql_parser::types::Value` to
`async_graphql_parser::types::ConstValue` since in 99% of cases in this library
a const value is wanted instead of a value.
- Added consistent usage of executable/service instead of the ambiguous
query/schema.
- Some of the tests actually had invalid GraphQL so the new more correct
grammar made them fail, that was fixed.
- Added a `Name` newtype to refer to GraphQL names
(`[A-Za-z_][A-Za-z_0-9]*`) since they are used so frequently.
Allow variables to be missing when the associated variable definition is
nullable (in which case we use `null` as the default).
This fixes queries like
```graphql
query Test($var: Int) {
test(var: $var)
}
```
```json
{}
```
which appear to be allowed according to the GraphQL spec.
