Aard custom (#49)

* Merge changes to docs

* Fix typo

* Correct SUMMARY so it compiles; update .gitignore

* Clean up statements.md

Make syntax and notation consistent with Rust source code.

* Fix statements for Merkle trees and compound types

* First draft of custom statements and small updates to signedpod.md

* Update book/src/merkletree.md

Co-authored-by: Ahmad Afuni <root@ahmadafuni.com>

* merklestatements correct typo

Co-authored-by: Ahmad Afuni <root@ahmadafuni.com>

* add todo for gadget ids

Co-authored-by: Ahmad Afuni <root@ahmadafuni.com>

* Separate out custom statements version 1

* More details on custom statements version 1

* new file custom2

* Partial draft of version 2

* First draft of version 2 spec, it's kind of a mess

* Another version of the custom predicates spec

* Update book/src/custom2.md

Co-authored-by: Eduard S. <eduardsanou@posteo.net>

* Simple example of deduction rule applied in circuit

* Implement Edu's comments on custom predicates

* Backend predicates must be defined in groups

* Add more examples

* Two diff statements using same constant

* Remove deprecated example

---------

Co-authored-by: Ahmad Afuni <root@ahmadafuni.com>
Co-authored-by: Eduard S. <eduardsanou@posteo.net>

.gitignore

@@ -2,3 +2,4 @@
 Cargo.lock
 .DS_Store
 aardnotes.md
+notes