add type alias with default rc impl

Dependencies

• [2] NW4S4ZCC rename MutTxn to GenericMutTxn
• [3] SO25TWFL A few features for integrating it into Pijul
• [4] E4MD6T3L Proofreading and commenting of this crate (massive bug fixes included)
• [5] OP6SVMOD Resetting history
• [6] YDHYZA77 Adding the RootPage trait to access the raw bytes of a root page (needed in libpijul)
• [7] YATBB4O7 run cargo fmt
• [8] W2MIZD5B Single file databases + CRC for the root pages (checking the other pages makes everything very slow)
• [*] WS4ZQM4R Debugging, tests, etc.

Change contents

• replacement in "sanakirja/src/lib.rs" at line 107
  [3.2997]→[3.2200:2267](∅→∅)
  − pub use environment::{Commit, Env, MutTxn, RootDb, RootPage, Txn};
  [3.2997]

  [3.2267]
  + pub use environment::{Commit, Env, GenericMutTxn, MutTxn, RootDb, RootPage, Txn};
• edit in "sanakirja/src/environment/muttxn.rs" at line 14
  [2.107]

  [10.37287]
  +

  + pub type MutTxn<E: Borrow<Env>, T> = GenericMutTxn<E, T, u64>;