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  There is a user-facing change here: things like `RUSTC_BOOTSTRAP=0` no
  longer active nightly features.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_resolve/src/late/diagnostics.rs

@@ -16,7 +16,6 @@ use rustc_hir::def::Namespace::{self, *};
 use rustc_hir::def::{self, CtorKind, CtorOf, DefKind};
 use rustc_hir::def_id::{DefId, CRATE_DEF_INDEX, LOCAL_CRATE};
 use rustc_hir::PrimTy;
-use rustc_session::config::nightly_options;
 use rustc_session::parse::feature_err;
 use rustc_span::hygiene::MacroKind;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
@@ -890,7 +889,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
             }
             (Res::Def(DefKind::TyAlias, def_id), PathSource::Trait(_)) => {
                 err.span_label(span, "type aliases cannot be used as traits");
-                if nightly_options::is_nightly_build() {
+                if self.r.session.is_nightly_build() {
                     let msg = "you might have meant to use `#![feature(trait_alias)]` instead of a \
                                `type` alias";
                     if let Some(span) = self.def_span(def_id) {
@@ -1675,7 +1674,7 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
                 _ => {}
             }
         }
-        if nightly_options::is_nightly_build()
+        if self.tcx.sess.is_nightly_build()
             && !self.tcx.features().in_band_lifetimes
             && suggests_in_band
         {