use expanded Analyzer for some validators for speed

[koron]

this may fix a part of #146

the slow validation is caused by many $refs in a YAML
See also an example spec file ref-1000.zip

the cause is that defaultValidator and exampleValidator don't use a common resolve cache.
so each $ref loads YAML file and it takes about 20msec in per a load.
therefore it tooks 20sec if YAML has 1000 $ref

in this patch, it uses s.expanded.Analyzer instead of s.analyer if it is available.
s.expanded and Analyzer are expected that be filled by validateReferencesValid which called by SpecValidator.Validate.

[koron]

i'm not sure that this is right to fix.
but in my case, significantly improved speed in my case: 28sec -> 4sec

[koron]

i modify more validators to use expanded analyzer.

[codecov]

Codecov Report

Merging #149 (cfc09bf) into master (c5b9514) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
- Coverage   90.70%   90.65%   -0.06%     
==========================================
  Files          22       22              
  Lines        2970     2975       +5     
==========================================
+ Hits         2694     2697       +3     
- Misses        224      225       +1     
- Partials       52       53       +1     

Impacted Files	Coverage Δ
default_validator.go	87.62% <100.00%> (ø)
example_validator.go	83.06% <100.00%> (ø)
helpers.go	82.24% <100.00%> (ø)
spec.go	87.08% <100.00%> (-0.24%)	⬇️

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[youyuanwu]

@casualjim Do we have some regression, or this is just a improvement?

[casualjim]

I think this was just an improvement.