Implement ToJson for &[T], and add tests. Closes #14619
syntax: Make quasiquoter use absolute paths

As part of removing `pub use` glob, two extra import globs were
injected to make `quote_expr!` work. However the globs caused
`unused_import` warning in some places.

Quasiquoter needed the globs since it generated idents (e.g. `TyU`)
rather than absolute paths (`::syntax::ast::TyU`).
This patch removes the extra globs and makes quasiquoter use absolute
paths.

Fixes #14618
auto merge of #14625 : japaric/rust/slice-tojson, r=alexcrichton

Let me know if the amount of tests is enough or too much.
auto merge of #14626 : klutzy/rust/issue-14618, r=alexcrichton

As part of removing `pub use` glob, two extra import globs were
injected to make `quote_expr!` work. However the globs caused
`unused_import` warning in some places.

Quasiquoter needed the globs since it generated idents (e.g. `TyU`)
rather than absolute paths (`::syntax::ast::TyU`).
This patch removes the extra globs and makes quasiquoter use absolute
paths.

Fixes #14618

cc @sfackler