Rename 'monoids' to 'semigroup'

[Insun35]

Issue type

Refactoring

Current behavior?

In this library, 'identities' is implemented separately from 'monoids'
Considering the definition of a monoid in abstract algebra, the currently implemented 'monoids' should be renamed as 'semigroups'.
Monoids: Monoids are semigroups with identity.
Semigroup: Semigroups may be considered a special case of magmas where the operation is associative, or as a generalization of groups, without requiring the existence of an identity element or inverses.

Expected Behavior?

Classification of libraries that fit the definition of abstract algebra.