Cleanup and issue a new release

[skx]

• Update the fuzzer for 1.18, now that is available.
• Fix the linter issues:

# github.com/skx/evalfilter/v2/lexer
lexer/lexer.go:305:4: declaration of "tok" shadows declaration at line 72
# github.com/skx/evalfilter/v2/vm
vm/vm.go:990:3: declaration of "field" shadows declaration at line 979
# github.com/skx/evalfilter/v2/parser
parser/parser.go:690:16: impossible condition: non-nil == nil
parser/parser.go:892:16: impossible condition: non-nil == nil
evalfilter_test.go:1369:3: don't use underscores in Go names; struct field foo_bar should be fooBar

[skx]

Fuzz-testing was updated in #178