Unify crates

[sanxiyn]

Fix #39.

Following mechanical changes were made:

• Delete Cargo.toml in subdirectories and update toplevel Cargo.toml.
• Rename lib.rs in subdirectories to mod.rs and move lint directives to toplevel lib.rs.
• Add mod declarations to toplevel lib.rs.
• Prefix use utils (and bindings, external_bindings, raftpb_bindings) with crate::.
• Except for implement_type_conversion, which is a macro.
• Also use $crate in a macro.
• Change use crate:: to use super::.
• Delete rust-toolchain.toml. (We are now on stable.)