shell.nix defines the install prefix as $(pwd)/inst

Includes documentation and test.

Works for uploading and not downloading.

floating-point numbers are supported now, including the fromJSON
builtin. Reported on IRC by inquisitiv3

This removes confusing documentation. It's better to remove doc than add implementation, because Nix 1.12 will surely have new GC interface anyway.

Fixes https://github.com/NixOS/nix/issues/641

Fixes #1298

While trying to understand garbage collection it was not immediately
clear that only the runtime dependency closure of output paths
would be kept (instead of the build-time dependency closure).
This commit attempts to clarify this by expanding some of the
glossary definitions and extending the Garbage Collection
section.

re-order the options using the alphabetical order and improve the example in the positive lookup case

Fixes #1981.

#1840

also document it

well