Add a new module 'Optional'
Add a new template constant 'nullable'
Add a new template constant 'comparable'
Add a new type 'Optional'
Add a new function 'hasValue'
Add a new function 'get'
Add a new function 'getOr'
Add three new functions 'optional'
Add unittest for 'Optional'
Merge branch 'module/optional' into dev refs #2