sort evaluated arguments in formals' order to ensure cache hit

[bluaze]

when the function has multiple default arguments or the default argument is not defined at the end of formal list, whether the default values are passed explicitly (by call) or implicitly (by default mechanism) may matter

fn <- function(x = 1, y = 1)
fm <- memoise(fn)

fm(x = 1)
fm(y = 1)

the last 2 call are essentially same, but would have called fn twice. that is because default values are attached to args ignoring their order in formals. sort them in original order would solve this.

[codecov-io]

Codecov Report

Merging #89 into master will decrease coverage by 16.24%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           master      #89       +/-   ##
===========================================
- Coverage   77.37%   61.13%   -16.25%     
===========================================
  Files           5        5               
  Lines         221      229        +8     
===========================================
- Hits          171      140       -31     
- Misses         50       89       +39

Impacted Files	Coverage Δ
R/memoise.R	100% <100%> (ø)	:arrow_up:
R/cache_s3.R	0% <0%> (-100%)	:arrow_down:

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