Add function to instantiate seeded MTRng

[emallson]

This PR consists of two parts:

1. Added a function mersenne() that creates an instance of MTRng seeded with the next u64 from the OsRng. This makes it easier to drop-in replace this library in code that uses thread_rng() and Rng-passing style (i.e. creating a single thread_rng() and then passing it to child functions rather than making new instances on-demand), because seeding is already handled (so it is again a one-liner instead of several lines and imports to generate the seed and then the MTRng instance). Taking the next random u64 is done instead of using e.g. the current time because this guarantees that even when multiple instances of a program start at once or when multiple calls to mersenne() are made simultaneously (e.g. one on each thread), they will have distinct seeds.

2. Added the remaining 910 values to the MT64 test vector. This was just something I did to triple-check the implementation. The test still passes with no changes to the implementation, this just makes the sequence of numbers used match the official test list.

(There are some other changes that are the result of rustfmt running automatically on the project due to my editor setup).