Book-length living documentation

Currently most of what you want is either in old papers or in the code; we should have a document that is continuously updated and cross-referenced so that a complete newcomer can get useful information from it.

Unlike most high efforts this can be done incrementally on master and will be immediately useful.

Quick thoughts on possible organization:

* Guide (task oriented)
* System requirements and dependencies
* Downloading the compiler
* Write, compile, and run code with the concrete syntax compiler
* Same but with the pure function translator; [other translators?]
* Writing FFI code
* Build from source, proofs, tests
* User reference
* Overview: motivation, design decisions
* Language semantics: values, types, expressions, declarations, basis
* Operating environment: targets (including references to asm semantics), FFI assumptions, machine setup assumptions, invocation of executables generated with basis_ffi
* Standalone compiler manual: concrete syntax, options index, system requirements and invocation
* Translator manual: what translators exist, detailed rules for what they accept, invocation of compilationLib, HOL setup requirements
* Optimization manual: exactly enough information about the passes to predict time and space complexity, optimization options, description of the explorer and its output
* Debugging manual: representation of values and frames for when you need an assembly debugger, runtime tracing options
* Developer reference
* Include language and asm semantics by reference from user manual
* Detailed semantics of all intermediate languages
* Detailed description of motivation, behavior and invariants of all passes