Closes #216.

This PR allows pallets to define a `view_functions` impl like so:

```rust
#[pallet::view_functions]
impl<T: Config> Pallet<T>
where
	T::AccountId: From<SomeType1> + SomeAssociation1,
{
	/// Query value no args.
	pub fn get_value() -> Option<u32> {
		SomeValue::<T>::get()
	}

	/// Query value with args.
	pub fn get_value_with_arg(key: u32) -> Option<u32> {
		SomeMap::<T>::get(key)
	}
}
```
### `QueryId`

Each view function is uniquely identified by a `QueryId`, which for this
implementation is generated by:

```twox_128(pallet_name) ++ twox_128("fn_name(fnarg_types) -> return_ty")```

The prefix `twox_128(pallet_name)` is the same as the storage prefix for pallets and take into account multiple instances of the same pallet.

The suffix is generated from the fn type signature so is guaranteed to be unique for that pallet impl. For one of the view fns in the example above it would be `twox_128("get_value_with_arg(u32) -> Option<u32>")`. It is a known limitation that only the type names themselves are taken into account: in the case of type aliases the signature may have the same underlying types but a different id; for generics the concrete types may be different but the signatures will remain the same.

The existing Runtime `Call` dispatchables are addressed by their concatenated indices `pallet_index ++ call_index`, and the dispatching is handled by the SCALE decoding of the `RuntimeCallEnum::PalletVariant(PalletCallEnum::dispatchable_variant(payload))`. For `view_functions` the runtime/pallet generated enum structure is replaced by implementing the `DispatchQuery` trait on the outer (runtime) scope, dispatching to a pallet based on the id prefix, and the inner (pallet) scope dispatching to the specific function based on the id suffix.

Future implementations could also modify/extend this scheme and routing to pallet agnostic queries.

### Executing externally

These view functions can be executed externally via the system runtime api:

```rust
pub trait ViewFunctionsApi<QueryId, Query, QueryResult, Error> where
	QueryId: codec::Codec,
	Query: codec::Codec,
	QueryResult: codec::Codec,
	Error: codec::Codec,
{
	/// Execute a view function query.
fn execute_query(query_id: QueryId, query: Query) -> Result<QueryResult,
Error>;
}
```
### `XCQ`
Currently there is work going on by @xlc to implement [`XCQ`](https://github.com/open-web3-stack/XCQ/) which may eventually supersede this work.

It may be that we still need the fixed function local query dispatching in addition to XCQ, in the same way that we have chain specific runtime dispatchables and XCM.

I have kept this in mind and the high level query API is agnostic to the underlying query dispatch and execution. I am just providing the implementation for the `view_function` definition.

### Metadata
Currently I am utilizing the `custom` section of the frame metadata, to avoid modifying the official metadata format until this is standardized.

### vs `runtime_api`
There are similarities with `runtime_apis`, some differences being:
- queries can be defined directly on pallets, so no need for boilerplate declarations and implementations
- no versioning, the `QueryId` will change if the signature changes. 
- possibility for queries to be executed from smart contracts (see below)

### Calling from contracts
Future work would be to add `weight` annotations to the view function queries, and a host function to `pallet_contracts` to allow executing these queries from contracts.

### TODO

- [x] Consistent naming (view functions pallet impl, queries, high level api?)
- [ ] End to end tests via `runtime_api`
- [ ] UI tests
- [x] Mertadata tests
- [ ] Docs

---------

Co-authored-by: kianenigma <kian@parity.io>
Co-authored-by: James Wilson <james@jsdw.me>
Co-authored-by: Giuseppe Re <giuseppe.re@parity.io>
Co-authored-by: Guillaume Thiolliere <guillaume.thiolliere@parity.io>