Prepare for inclusion into libstd

This applies the same change as gimli-rs/gimli#503 to this crate.

.travis.yml

@@ -25,7 +25,7 @@ addons:
     - libssl-dev
 
 script:
-  - cargo build --all-features && cargo test --all-features && cargo doc --all-features
+  - cargo build --features all && cargo test --features all && cargo doc --features all
   - cargo build --no-default-features --features read
   - cargo build --no-default-features --features write
   - cargo build --no-default-features --features read_core,write_core,coff

Cargo.toml

@@ -18,6 +18,12 @@ flate2 = { version = "1", optional = true }
 indexmap = { version = "1.1", optional = true }
 wasmparser = { version = "0.54", 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 }
+alloc = { version = '1.0.0', optional = true, package = 'rustc-std-workspace-alloc' }
+
 [dev-dependencies]
 memmap = "0.7"
 
@@ -38,6 +44,14 @@ wasm = ["wasmparser"]
 
 default = ["read", "compression"]
 
+# Umbrella feature for enabling all user-facing features of this crate. Does not
+# enable internal features like `rustc-dep-of-std`.
+all = ["read", "write", "std", "compression", "default"]
+
+# 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', 'alloc']
+
 [[example]]
 name = "nm"
 required-features = ["read"]