Make Jsonnet std lib non-global

[lihaoyi-databricks]

This serves two purposes:

1. We think we're seeing race conditions where the lazily-evaluated mutable state in the std lib Val.Obj is causing livelocks when evaluating Jsonnet in a multithreaded environment.

2. This will allow whoever instantiates sjsonnet.Interpreter to customize the std.*, allowing people to inject their own functions. Useful if google/jsonnet has some new std lib functions that haven't been added to sjsonnet yet, or if you have some domain-specific functions you want to expose in your config

[lihaoyi-databricks]

Review by @szeiger

[szeiger]

I think in the longer term we should look into getting a clearer separation of official Jsonnet functions and custom extensions instead of putting non-standard functions into std.

[lihaoyi-databricks]

Yeah this change would open up the possibility of defining custom std functions like gzip or xz in the databricks codebase, and other people can define their own std functions easily, rather than having to chuck everything into the shared Std.scala

[szeiger]

What I meant is that we should put them into a separate object (e.g. stdext). I think we already have some non-standard features in std. This makes it hard to tell if a source is compatible with Jsonnet or if it requires non-standard Sjsonnet features.