Allow making RUSTC_BOOTSTRAP conditional on the crate name

The main change is that `UnstableOptions::from_environment` now requires
an (optional) crate name. If the crate name is unknown (`None`), then the new feature is not available and you still have to use `RUSTC_BOOTSTRAP=1`. In practice this means the feature is only available for `--crate-name`, not for `#![crate_name]`; I'm interested in supporting the second but I'm not sure how.

Other major changes:

- Added `Session::is_nightly_build()`, which uses the `crate_name` of
the session
- Added `nightly_options::match_is_nightly_build`, a convenience method
for looking up `--crate-name` from CLI arguments.
`Session::is_nightly_build()`should be preferred where possible, since
it will take into account `#![crate_name]` (I think).
- Added `unstable_features` to `rustdoc::RenderOptions`

  There is a user-facing change here: things like `RUSTC_BOOTSTRAP=0` no
  longer active nightly features. In practice this shouldn't be a big
  deal, since `RUSTC_BOOTSTRAP` is the opposite of stable and everyone
  uses `RUSTC_BOOTSTRAP=1` anyway.

- Add tests

  Check against `Cheat`, not whether nightly features are allowed.
  Nightly features are always allowed on the nightly channel.

- Only call `is_nightly_build()` once within a function

- Use booleans consistently for rustc_incremental

  Sessions can't be passed through threads, so `read_file` couldn't take a
  session. To be consistent, also take a boolean in `write_file_header`.

compiler/rustc_passes/src/check_const.rs

@@ -15,7 +15,6 @@ use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
 use rustc_middle::hir::map::Map;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::TyCtxt;
-use rustc_session::config::nightly_options;
 use rustc_session::parse::feature_err;
 use rustc_span::{sym, Span, Symbol};
 
@@ -145,7 +144,7 @@ impl<'tcx> CheckConstVisitor<'tcx> {
                 //
                 // FIXME(ecstaticmorse): Maybe this could be incorporated into `feature_err`? This
                 // is a pretty narrow case, however.
-                if nightly_options::is_nightly_build() {
+                if tcx.sess.is_nightly_build() {
                     for gate in missing_secondary {
                         let note = format!(
                             "add `#![feature({})]` to the crate attributes to enable",