std/sandbox

Loads .trb files as sandboxed receiver closures - the mechanism behind project.trb and configuration scripts (see "Receiver Scripts and the Sandbox" in CONCEPT.md, and docs/design/SCRIPTS.md).

A script runs in the VM, always: the step and time limits and the panic that does not end the caller are properties of an interpreter (docs/design/SCRIPTS.md section 1). A program torb run runs in the VM loads one, from a path it was compiled with or from any other, and so does a native binary torb build --embed-vm built, which embeds the VM. A native binary torb build compiled loads one through the front end and the VM it embeds (slice 8): the script is checked against Value and run in that VM, and the value crosses as text in both directions, through the derived Encode and Decode of Value, every field included - so a receiver of such a program is Encode & Decode, which a type of plain settings is by derivation.