Add initial coredump document

Closes #190.

Coredump.md

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+This document describes the usage of coredump for post-mortem debugging with
+WebAssembly.
+
+Stability of this specification: **experimental**.
+
+# Idea
+
+When WebAssembly enters a trap, it starts unwinding and collects debugging
+information. For each stack frame, the state of the WebAssembly instance can be
+captured by collecting the values:
+- in locals (includes function parameters)
+- in globals
+- on the stack
+
+Along with an instruction binary offset (relative to the code section, as
+specified by [DWARF]).
+
+The post mortem analyze is done using the coredump file (that includes the
+collected values and the WebAssembly linear memory) and [DWARF] informations.
+Similar to the debugging flow with [gdb].
+
+## Runtime support
+
+Most of the WebAssembly runtimes are already able to present a useful stacktrace
+on crash to the user. When they are configured to emit a coredump, they collect
+the debugging information and write a coredump file.
+
+An example output:
+```
+$ wasmrun module.wasm
+
+Exit 1: Uncaught RuntimeError: memory access out of bounds (core dumped).
+``` 
+A coredump file has been generated.
+
+For experimenting, runtime support is not strictly necessary. A tools can
+transform the Wasm binary to inject code that will manually unwind the stack and
+collect debugging information, for instance [wasm-edit coredump]. Such a
+transformation has important limitations; a trap caused by an invalid memory
+operation or exception in a host function might not be caught.
+
+## Security and privacy considerations
+
+Using the WebAssembly linear memory for debugging exposes the risk of seeing,
+manipulating and/or collecting sensitive informations. Handling of sensitive
+data is out of scope.
+
+No particular security considerations.
+
+## Debugger support
+
+[gdb] doesn't support Wasm coredump and it's unclear if it can. Wasm coredump
+differ from ELF coredump in a few significant ways:
+- Wasm semantics; usage of locals, globals and the stack.
+- The process image is only the Wasm linear memory.
+- etc.
+
+For experimenting, a custom tool has been built and mimics [gdb]: [wasmgdb].
+
+# Coredump file format
+
+The generated coredump is a binary file containing: 
+- a snapshot of the WebAssembly linear memory or relevant regions.
+- the `coredump` struct.
+
+The placement of the `coredump` struct within the coredump file is not defined
+yet. However, for the sake of argument we assume it's at offset 0.
+
+# `coredump` struct
+
+The coredump struct starts with the numbers of frame recorded and the combined
+size of all frames, followed by the frames themself.
+
+```
+coredump ::= size:u32 cont:vec(frame)
+```
+
+> implementer note: since the `frame` struct doesn't have a fixed size, the
+> `size` value can be used to append new frames because it's pointing at the end
+> of the `coredump`.
+
+```
+frame ::= codeoffset:u32
+          locals:vec(local)
+          globals:vec(global)
+          stack:vec(stack)
+          reserved:u32
+```
+
+The `reserved` bytes are decoded as an empty vector and reserved for future use.
+
+`vec` (same encoding as [Wasm Vectors]):
+```
+vec(B) ::= n:u32 cont:B
+```
+
+`u32` are encoding using LEB128, like [Wasm u32].
+
+# Demo
+
+Please have a look at the demonstration using the experimental support and
+tooling: [demo].
+
+# Useful links
+
+- [ELF coredump]
+- [Wasmer FrameInfo]
+
+[Wasm Vectors]: https://webassembly.github.io/spec/core/binary/conventions.html#binary-vec
+[ELF coredump]: https://www.gabriel.urdhr.fr/2015/05/29/core-file/
+[Core dump on Wikipedia]: https://en.wikipedia.org/wiki/Core_dump
+[gdb]: https://linux.die.net/man/1/gdb
+[wasm-edit coredump]: https://github.com/xtuc/wasm-edit/blob/main/src/coredump.rs
+[wasm-edit]: https://github.com/xtuc/wasm-edit
+[wasmgdb]: https://github.com/xtuc/wasmgdb
+[DWARF]: https://yurydelendik.github.io/webassembly-dwarf
+[Wasmer FrameInfo]: https://docs.rs/wasmer/latest/wasmer/struct.FrameInfo.html
+[Wasm u32]: https://webassembly.github.io/spec/core/binary/values.html#binary-int
+[demo]: https://github.com/xtuc/wasmgdb/wiki/Demo