Improve default for durations

[riptl]

Summary: This improves support for the default struct tag on time.Duration.

Problem

time.Duration is an int64 type, so it's already supported by the default tag.
However, the default is interpreted as a number of nanoseconds, not a duration, making parsing a bit awkward.

Example for 120ms:

var doc struct {
    DurationField time.Duration `default:"120000000"`
}

Feature

Go's time library supports a human-readable notation for durations.
Including this improves readability:

var doc struct {
    DurationField time.Duration `default:"120ms"`
}

Implementation notes

To preserve backwards-compatibility, the code checks for three conditions:

• The type is int64
• The type name is time.Duration
• The last char of the provided default is not a number

If either is not met, the code falls back to parsing a number.

Open questions:

• Is the backwards-compatibility check overkill, or good means of preventing people from shooting themselves in the foot?
• Instead of checking .Type().String(), is it preferred to check for .Type().PkgName() and .Type().Name() for a tiny bit more performance?

[codecov]

Codecov Report

Merging #464 (69117ae) into master (ba1b12b) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #464      +/-   ##
==========================================
+ Coverage   93.60%   93.62%   +0.02%     
==========================================
  Files          10       10              
  Lines        2266     2274       +8     
==========================================
+ Hits         2121     2129       +8     
  Misses        105      105              
  Partials       40       40              

Impacted Files	Coverage Δ
marshal.go	95.91% <100.00%> (+0.05%)	⬆️

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[pelletier]

Looks good, thank you for the patch!