Switch to SPDX 2.1 license expression

According to the Cargo Reference:
https://doc.rust-lang.org/cargo/reference/manifest.html

> This is an SPDX 2.1 license expression for this package. Currently
> crates.io will validate the license provided against a whitelist of
> known license and exception identifiers from the SPDX license list
> 2.4. Parentheses are not currently supported.
>
> Multiple licenses can be separated with a `/`, although that usage
> is deprecated. Instead, use a license expression with AND and OR
> operators to get more explicit semantics.

Author: dtolnay

src/libstd/Cargo.toml

@@ -3,7 +3,7 @@ authors = ["The Rust Project Developers"]
 name = "std"
 version = "0.0.0"
 build = "build.rs"
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 repository = "https://github.com/rust-lang/rust.git"
 description = "The Rust Standard Library"
 edition = "2018"

src/tools/rustbook/Cargo.toml

@@ -2,7 +2,7 @@
 authors = ["The Rust Project Developers"]
 name = "rustbook"
 version = "0.1.0"
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 edition = "2018"
 
 [dependencies]

src/tools/rustc-std-workspace-alloc/Cargo.toml

@@ -2,7 +2,7 @@
 name = "rustc-std-workspace-alloc"
 version = "1.0.0"
 authors = ["Alex Crichton <[email protected]>"]
-license = 'MIT/Apache-2.0'
+license = 'MIT OR Apache-2.0'
 description = """
 Hack for the compiler's own build system
 """

src/tools/rustc-std-workspace-core/Cargo.toml

@@ -2,7 +2,7 @@
 name = "rustc-std-workspace-core"
 version = "1.0.0"
 authors = ["Alex Crichton <[email protected]>"]
-license = 'MIT/Apache-2.0'
+license = 'MIT OR Apache-2.0'
 description = """
 Hack for the compiler's own build system
 """

src/tools/rustc-workspace-hack/Cargo.toml

@@ -2,7 +2,7 @@
 name = "rustc-workspace-hack"
 version = "1.0.0"
 authors = ["Alex Crichton <[email protected]>"]
-license = 'MIT/Apache-2.0'
+license = 'MIT OR Apache-2.0'
 description = """
 Hack for the compiler's own build system
 """

src/tools/unstable-book-gen/Cargo.toml

@@ -3,7 +3,7 @@ authors = ["est31 <[email protected]>",
            "The Rust Project Developers"]
 name = "unstable-book-gen"
 version = "0.1.0"
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 edition = "2018"
 
 [dependencies]