Export Failpoint/Failpoints as public types

[shafreeck]

What problem does this PR solve?

The failpoint package is not flexible enough in use cases like

• Using in the unit tests
• Building a mock library
• As a core of other fail point implementations
• Making a service fail fast
• Chaos Engineering

What is changed and how it works?

• Make Failpointa public type
• Make Failpoints a public type
• Return an error when calling Eval
• Remove the failpath field from term struct.

A proposal is available in Chinese: https://docs.google.com/document/d/1OUheSlxWkQZqIb8mLZYKE8pxlSc1YElXFutjnPQe5UE/edit?usp=sharing

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

Side effects

• Breaking backward compatibility

The signature of Eval is changed.

[codecov[bot]]

Codecov Report

Merging #50 into master will decrease coverage by 0.0015%. The diff coverage is 84.8167%.

@@               Coverage Diff                @@
##             master        #50        +/-   ##
================================================
- Coverage   85.7453%   85.7438%   -0.0016%     
================================================
  Files             7          7                
  Lines           919        968        +49     
================================================
+ Hits            788        830        +42     
- Misses           82         85         +3     
- Partials         49         53         +4     

[shafreeck]

@kennytm @lonng PTAL

[shafreeck]

@kennytm @lonng All issues have been fixed.

[shafreeck]

@kennytm @lonng The lock problem was fixed.