Collect previous design documents in proposals/stack-switching. (#96)

This patch collects the documents of `proposals/bag-o-stacks` and `proposals/continuations` directories into `proposals/stack-switching/design-notes`. The examples in `proposals/continuations/examples` have been copied over to `proposals/stack-switching/examples`. In addition the `proposals/stack-switching-requirements/requirements.md` has been moved to `proposals/stack-switching/design-notes/requirements.md`.

The `README.md` has been updated as well.

Author: dhil

README.md

@@ -1,35 +1,13 @@
+[![CI for specs](https://github.com/WebAssembly/stack-switching/actions/workflows/ci-spec.yml/badge.svg)](https://github.com/WebAssembly/stack-switching/actions/workflows/ci-spec.yml)
+[![CI for interpreter & tests](https://github.com/WebAssembly/stack-switching/actions/workflows/ci-interpreter.yml/badge.svg)](https://github.com/WebAssembly/stack-switching/actions/workflows/ci-interpreter.yml)
+
 # Stack-Switching Proposal for WebAssembly
 
 This repository is a clone of [`WebAssembly/spec`](https://github.com/WebAssembly/spec/). It is meant for discussion, prototype specification, and implementation of a proposal to add
 support for stack-switching.
 
 See the [explainer](proposals/stack-switching/Explainer.md) for a high-level summary of the proposal.
 
-## Previous proposals
-
-The current explainer represents the unification of two previous proposals: Typed Continuations (wasmfx) and Bag of Stacks (bos). (The explainers have now been unified. Once the reference interpreter and examples are adapted for the unified proposal this section will be removed from the README.)
-
-#### Typed Continuations
-
-* See the [explainer](proposals/continuations/Explainer.md) for a high-level summary of the proposal.
-
-* See the [overview](proposals/continuations/Overview.md) for a more formal description of the proposal.
-
-* An [implementation](https://github.com/WebAssembly/stack-switching/tree/wasmfx) is available as an extension to the reference interpreter. It is accesible from the `wasmfx` branch of this repository.
-
-* See the [examples](proposals/continuations/examples) for Wasm code for implementing various different features including lightweight threads, actors, and async/await.
-
-#### Bag of Stacks Proposal
-
-* See the [explainer](proposals/bag-o-stacks/Explainer.md) for a high-level summary of the proposal.
-
-Original README from upstream repository follows.
-
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-
-[![CI for specs](https://github.com/WebAssembly/stack-switching/actions/workflows/ci-spec.yml/badge.svg)](https://github.com/WebAssembly/stack-switching/actions/workflows/ci-spec.yml)
-[![CI for interpreter & tests](https://github.com/WebAssembly/stack-switching/actions/workflows/ci-interpreter.yml/badge.svg)](https://github.com/WebAssembly/stack-switching/actions/workflows/ci-interpreter.yml)
-
 # spec
 
 This repository holds the sources for the WebAssembly specification,