getstatic can support singleton fields now

[Hearen]

#101

add a singleton to demo.Demo as:

static class MySingleton {
        private static MySingleton singleton = new MySingleton();
        public String val = "it's me, yo";

        private MySingleton() {

        }

        public static MySingleton getInstance() {
            return singleton;
        }
    }

$ getstatic *Demo$MySingleton val
field: singleton
@MySingleton[
    singleton=@MySingleton[demo.Demo$MySingleton@5ae97585],
    val=@String[it's me, yo],
]
Affect(row-cnt:1) cost in 10645 ms.

[codecov-io]

Codecov Report

Merging #230 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@           Coverage Diff            @@
##           master   #230      +/-   ##
========================================
- Coverage    4.61%   4.6%   -0.01%     
========================================
  Files         190    190              
  Lines        9347   9353       +6     
  Branches     1620   1623       +3     
========================================
  Hits          431    431              
- Misses       8873   8879       +6     
  Partials       43     43

Impacted Files	Coverage Δ
...arthas/core/command/klass100/GetStaticCommand.java	0% <0%> (ø)	⬆️

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[hengyunabc]

What the user needs is to call the static method of the static class. Refer to #240

[Hearen]

What the user needs is to call the static method of the static class. Refer to #240

@hengyunabc I am referring to this issue: #101