More debugger work.

Posix only right now.

docs/debugger.md

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+# Debugger
+
+## Protocol
+
+Framed messages:
+
+```
+[4b type] (0=request, 1=notification, etc)
+[4b content source id]
+[4b request/response id]
+[4b size]
+[payload]
+```
+
+## Content Sources
+
+### `xe::dbg::sources::MemorySource`
+
+[ ] Paged view into memory
+[ ] Search operations
+[ ] Live streaming updates
+[ ] Writes
+
+### `xe::dbg::sources::ProcessorSource`
+
+[ ] Thread list
+[ ] State read/write (per thread)
+[ ] Modules
+[ ] Statistics
+[ ] Basic control (pause/resume)
+[ ] Breakpoints/checkpoints/traces
+[ ] Trace stream
+
+### `xe::dbg::sources::ModuleSource`
+
+[ ] Paged view into all symbols
+[ ] Get function contents (data, disasm, llvm, x86, etc)