Rewrite wrapper types to use downcasting approach and cleanup duplica…

[stewbis]

Wrapper code has become muddle with multiple inline transformations with inconsistent behavior. This change throws it all away and replaces it with a consistent approach whereby there are

• Converters, which convert from temporal type A -> B
• Providers, which provide a temporal type B
• Functions, which perform date functions on type B

All matchers apply a "down-casting" approach whereby the actual type is cast to the expected type before comparing. Invalid down-casts i.e. up-casts of say LocalDate to LocalDateTime are not supported because it leads to dubious tests whereby missing information such as time needs to be made up

[coveralls]

Coverage increased (+0.6%) to 88.262% when pulling 8d83580 on rewrite_wrappers into 703234d on master.