Add arg checking helper

[st0012]

• Add vm.checkArgTypes for checking built-in methods' argument types
• Replace most of the manual type checkings with the new helper
• Some refactorings for reducing local variable usages

[codecov[bot]]

Codecov Report

Merging #838 into master will increase coverage by 0.02%. The diff coverage is 89.61%.

@@            Coverage Diff             @@
##           master     #838      +/-   ##
==========================================
+ Coverage   81.08%   81.11%   +0.02%     
==========================================
  Files          54       54              
  Lines        7434     7412      -22     
==========================================
- Hits         6028     6012      -16     
+ Misses       1182     1178       -4     
+ Partials      224      222       -2

Impacted Files	Coverage Δ
vm/go_object.go	15.38% <0%> (ø)	:arrow_up:
vm/file.go	86.74% <100%> (+0.08%)	:arrow_up:
vm/string.go	98.26% <100%> (-0.01%)	:arrow_down:
vm/json.go	85.29% <100%> (ø)	:arrow_up:
vm/go_map.go	82.14% <100%> (-0.32%)	:arrow_down:
vm/concurrent_hash.go	94.69% <100%> (-0.31%)	:arrow_down:
vm/vm.go	83.07% <100%> (+0.53%)	:arrow_up:
vm/regexp.go	75.51% <33.33%> (ø)	:arrow_up:
vm/http_client.go	60% <50%> (+1.9%)	:arrow_up:
vm/class.go	86.2% <82.92%> (-0.09%)	:arrow_down:
... and 3 more

---

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 c642dbc...220949f. Read the comment docs.