Remove Index type from Config trait (#1074)

* remove config, doc tests are expected to fail (book not adjusted yet)

* make doc tests pass

* Prevent bug when reusing type ids in hashing (#1075)

* practice TDD

* implement a hashmap 2-phases approach

* use nicer types

* add test for cache filling

* adjust test

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Co-authored-by: James Wilson <james@jsdw.me>

* remove the unnecessary intos

---------

Co-authored-by: James Wilson <james@jsdw.me>

subxt/examples/setup_client_custom_config.rs

@@ -7,8 +7,6 @@ use subxt::{
 /// more information about each type):
 enum MyConfig {}
 impl Config for MyConfig {
-    // This is different from the default `u32`:
-    type Index = u64;
     // We can point to the default types if we don't need to change things:
     type Hash = <SubstrateConfig as Config>::Hash;
     type Hasher = <SubstrateConfig as Config>::Hasher;