Bump version to 0.5.1

The licence was updated to reflect the use of the fastrand crate being
used under the MIT license.

The README.md ws edited to include a mention of Spinoza (another Rust
based quantum computer simulator) and added some mentions about the
possible use of the custom function implementing non-unitary
transformations.

Several instances of ProductState::to_string being called explicilty in
print macros were removed in examples and the quick start guide.

Signed-off-by: Andrew Barlow <a.barlow.dev@gmail.com>

Improves the code of inserting states, where now the place of the states
in the hashmap will be stored for mutating.

Signed-off-by: Andrew Barlow <a.barlow.dev@gmail.com>

ProductState now has the IterTrait implemented so when invoked, it
returns an iter that cycles through the qubits that label the product
state in the computational basis.

Signed-off-by: Andrew Barlow <a.barlow.dev@gmail.com>

The user can now retrieve if the circuit will be printing explicit
simulation progress to the console.

Signed-off-by: Andrew Barlow <a.barlow.dev@gmail.com>

Feat state insertion

Please note that the base of this branch is from master and was realise too late; this was a mistake.

Bump the cargo version, and re-wrote the description for crates.io.

Signed-off-by: Andrew Barlow <a.barlow.dev@gmail.com>