Use a template free shim around quarterly.h classes

[DavisVaughan]

To greatly improve compilation speed

Basically, quarterly-year-quarter-day.cpp needed every function to be templated on both the precision and the quarter start, which resulted in a massive explosion in the amount of template code that had to be generated.

We don't really need templating on the quarter start, even though using a template in quarterly.h is technically correct (because it is part of the type definition). So this introduces a minimal shim around those classes that re-exposes them in a template free way.

The shim doesn't expose everything from quarterly.h, only what is needed for the R API. We can expose more as needed.

Essentially the template parameter is now a class member quarterly_shim::year. We went:

// from
quarterly::year<start>

// to
quarterly_shim::year.start()

On my 2018 Intel Mac, compilation time went down from 70s to 25s!

The internal switch over the quarter start types does make the quarterly type a little slower for some operations, but I think it is worth it:

library(clock)
set.seed(123)

q <- sample(4, 1e7, TRUE)
x <- year_quarter_day(2019, q)

bench::mark(set_day(x, "last"), iterations = 100)

# Before
#> # A tibble: 1 × 6
#>   expression              min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>         <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 set_day(x, "last")   77.9ms   81.4ms      12.0    38.2MB     8.01

# After
#> # A tibble: 1 × 6
#>   expression              min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>         <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 set_day(x, "last")   99.1ms    103ms      9.51    38.2MB     6.34