Adds debugging functionality

[DavZim]

This PR adds a debug argument to memoise, if set to TRUE, each function call will print out information about the function, arguments, key, time, as well as file size (if cachem::cache_disk is used).

I am not sure if this is something you want to have in the package, but the code was too helpful for me to drop it.
For me this helped me to debug a complex shiny application and find bottlenecks where the caching/memoisation were actually harming the overall performance.

The PR adds no dependencies and if left at its default (FALSE), it will not interfere with the existing functionality. If set to TRUE, it will print out the debug information, see for example

sleeper <- function(x) {
  Sys.sleep(x)
  x
}

memF <- memoise::memoise(sleeper, debug = TRUE)

memF(1)
#> Calling Function memF(x = 1)
#>  hash time: 0.0000s
#>  key: 4147be8bbfbabec5e01b4df9d18936be
#>  = no cache found
#>      eval time: 1.0186s
#>  ==> total time: 1.0186s
#> [1] 1
memF(1)
#> Calling Function memF(x = 1)
#>  hash time: 0.0010s
#>  key: 4147be8bbfbabec5e01b4df9d18936be
#>  = cache found
#>      cache read time: 0.0000s
#>  ==> total time: 0.0000s
#> [1] 1


memFF <- memoise::memoise(sleeper, cache = cachem::cache_disk(), debug = TRUE)
memFF(1)
#> Calling Function memFF(x = 1)
#>  hash time: 0.0000s
#>  key: 4147be8bbfbabec5e01b4df9d18936be
#>  = no cache found
#>      eval time: 1.0135s
#>      cache save time: 0.0120s
#>  new cache size: 102 bytes
#>  ==> total time: 1.0275s
#> [1] 1
memFF(1)
#> Calling Function memFF(x = 1)
#>  hash time: 0.0000s
#>  key: 4147be8bbfbabec5e01b4df9d18936be
#>  = cache found
#>      cache read time: 0.0040s
#>  ==> total time: 0.0040s
#> [1] 1

^{Created on 2022-12-07 by the reprex package (v2.0.1)}

Feel free to change the code if you feel it is missing something or is doing too much!