Subxt Guide (#890)

* WIP Starting to write book; extrinsics first pass done

* cargo fmt

* Ongoing work; events, constants, wip blocks

* at_latest() and wip blocks

* remove need to import parity-scale-codec crate with Subxt for macro to work

* More docs; expanding on setup guide and finish pass of main sections

* Tidy and remove example section for now

* format book lines to 100chars

* Fix example code

* cargo fmt

* cargo fmt

* fix example

* Fix typos

* fix broken doc links, pub mods

* Update Subxt macro docs

* can't link to Subxt here

* move macro docs to Subxt to make linking better and fix example code

* note on macro about docs

* cargo fmt

* document the no_default_derives macro feature

* Address feedback and remove redundant text

* address review comments; minor tweaks

* WIP add Runtime calls to book

* Improve Runtime API docs

* expose thing we forgot to expose and doc link fixes

codegen/src/api/mod.rs

@@ -225,8 +225,13 @@ impl RuntimeGenerator {
                 // Preserve any Rust items that were previously defined in the adorned module
                 #( #rust_items ) *
 
-                // Make it easy to access the root via `root_mod` at different levels:
-                use super::#mod_ident as root_mod;
+                // Make it easy to access the root items via `root_mod` at different levels
+                // without reaching out of this module.
+                #[allow(unused_imports)]
+                mod root_mod {
+                    pub use super::*;
+                }
+
                 #types_mod
             }
         })