refactor and unit test switch.go; various other changes

[nishanths]

Switch.go refactor and testing

Refactor code in switch.go to be unit-testable. Add some unit tests in switch_test.go.

Deprecate -ignore-pattern flag

The -ignore-pattern flag is deprecated and has no effect. Add -ignore-enum-members,
which has the same behavior, instead. Update the docs accordingly.

Support checking strategies

The new -checking-strategy flag can be used to specify a checking strategy, one of
"value" or "name". The default strategy is "value", which retains the existing behavior,
of requiring every enum member value to be listed in a switch statement.
The "name" strategy requires every enum member name to be listed in a switch
statement.

Add related docs in doc.go.

Add tests for this change in the testdata packages "duplicateenumvalue/...".

Analyzer does not suggest fixes

The "-fix" flag is now a no-op. The analyzer no longer reports any suggested fixes.
Also remove source code and testdata related to this flag. Update doc.go
accordingly.

CI

Include Go 1.14 explicitly in CI matrix. (This is to match with the go version in go.mod.)

Repo documentation

Add a separate CONTRIBUTING file. General corrections and updates to README.

Other changes

Improve gob-compatibility testing of fact types.