Suggested improvements to the language manual

[pmundkur]

The below constructs are under-discussed or missing in the manual:

• register (this is mentioned in the Tutorial portion and elsewhere, and is probably obvious, but a core construct like this deserves its own dedicated section in the Language section)
• assert (the manual uses assert liberally without really specifying what it does)
• instantiation ... with (this shows up in prelude_mem.sail in the RISC-V model)
• pure and impure (used as FFI annotations in the RISC-V model)
• foreach : (the grammar indicates there are also foreach ... by and foreach ... by .. in flavors; are they useful?)

[Alasdair]

Yes, thanks for creating an issue to track these.

I wonder if this would be a good time to use the Create sub-issue button I now see on Github?