cargo: add metadata to build as a dep of libstd

This commit adds the necessary `Cargo.toml` directives which enables
this crate to build with the standard library. This introduces some
optional off-by-deault dependencies which are only activated (and
overwritten) when built as part of the standard library.

Ref rust-lang/backtrace-rs#432

PR #89

.github/workflows/ci.yml

@@ -1,6 +1,8 @@
 name: ci
 on:
   pull_request:
+    branches:
+    - master
   push:
     branches:
     - master

Cargo.toml

@@ -31,9 +31,18 @@ std = []
 # then, it is alias for the 'std' feature.
 use_std = ["std"]
 
+# Internal feature, only used when building as part of libstd, not part of the
+# stable interface of this crate.
+rustc-dep-of-std = ['core', 'compiler_builtins']
+
 [dependencies]
 libc = { version = "0.2.18", default-features = false, optional = true }
 
+# Internal feature, only used when building as part of libstd, not part of the
+# stable interface of this crate.
+core = { version = '1.0.0', optional = true, package = 'rustc-std-workspace-core' }
+compiler_builtins = { version = '0.1.2', optional = true }
+
 [dev-dependencies]
 quickcheck = { version = "1.0.3", default-features = false }