Set manifest keywords and categories (#1042)

Keywords get the second-highest priority in crates.io search,
after crate name; we should set keywords.

Cargo.toml

@@ -18,6 +18,8 @@ name = "zerocopy"
 version = "0.8.0-alpha.7"
 authors = ["Joshua Liebow-Feeser <[email protected]>"]
 description = "Utilities for zero-copy parsing and serialization"
+categories = ["embedded", "encoding", "no-std::no-alloc", "parsing", "rust-patterns"]
+keywords = ["cast", "convert", "transmute", "transmutation", "type-punning"]
 license = "BSD-2-Clause OR Apache-2.0 OR MIT"
 repository = "https://github.com/google/zerocopy"
 rust-version = "1.56.0"