Bind the standard library to a `$std` variable and use it in desugared expressions

[eduardosm]

This keeps redefining std from breaking expressions that desugar to calls to standard library functions by redefining std.

This is similar to what the Go implementation does.

Fixes https://github.com/google/jsonnet/issues/1128.

[sbarzowski]

Thank you! This makes it cleaner.