evaluate default args in a dedicated environment

[bluaze]

each default arguments not called is assigned to a dedicated environment as promise, just like how R initialize evaluation frame of function call, and the enclosure of the environment is set to the enclosure of the function. then values of default arguments are retrieved from the environment.

would fix #64

[codecov-io]

Codecov Report

Merging #88 into master will decrease coverage by 16.58%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           master      #88       +/-   ##
===========================================
- Coverage   77.37%   60.79%   -16.59%     
===========================================
  Files           5        5               
  Lines         221      227        +6     
===========================================
- Hits          171      138       -33     
- 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:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 1650ad7...a7a7420. Read the comment docs.