ENH: vector methods for conversion from Period to its ordinal and back
#### Is your feature request related to a problem?

a) Currently to get Series of ordinal of Period Series I do


`s.apply(lambda  x: x.ordinal)

b) Also to get back I do

`s.apply(lambda x: Period(ordinal=x, 'W-SAT'))

c) And last but not least:

s.astype(int).astype('period[W-SAT]') is different than s.

#### Describe the solution you'd like

a)
s.ordinal  returns series with ordinals

b)
There is some easy method to create series of Periods from ordinals... like Period(s, 'W-SAT')
or s.astype('period[W-SAT]', from_ordinal=True)

c) last option in b) is kind of solution to c)


[this should provide a description of the feature request, e.g. "`DataFrame.foo` should get a new parameter `bar` that [...]", try to write a docstring for the desired feature]

#### API breaking implications

Mostly it won't but if .astype('period[W-SAT]') would use ordinals then yes as currently 2650 means year nor ordinal for this method


