fix advanced example in README

[GlenDC]

I'm also not sure if it's me making a mistake somewhere,
but the following line of code doesn't seem to work for me:

const findIndex = (c, d) => c - destringifyOffset.first().value;

Seems the first function is returning pretty much the same as what destringifyOffset
already gives me at that point. The following does work for me:

const findIndex = (c, d) => c - destringifyOffset.nodes[0].value;

Didn't modify that in the README example in this PR yet, as I do not know if it's a bug, desired or something I'm simply doing wrong.

[ism]

Hello everyone,

it is always discouraging when examples of something that is new to you doesn't work as expected.

I fixed advanced example to work for me like follows

``
const { refactor } = require('shift-refactor');
const Shift = require('shift-ast');
const { commonMethods } = require('refactor-plugin-common');

// Obfuscated source
const src = var a=['\x74\x61\x72\x67\x65\x74','\x73\x65\x74\x54\x61\x72\x67\x65\x74','\x77\x6f\x72\x6c\x64','\x67\x72\x65\x65\x74','\x72\x65\x61\x64\x65\x72'];var b=function(c,d){c=c-0x0;var e=a[c];return e;};(function(){class c{constructor(d){this[b('0x0')]=d;}['\x67\x72\x65\x65\x74'](){console['\x6c\x6f\x67']('\x48\x65\x6c\x6c\x6f\x20'+this[b('0x0')]);}[b('0x1')](e){this['\x74\x61\x72\x67\x65\x74']=e;}}const f=new c(b('0x2'));f[b('0x3')]();f[b('0x1')](b('0x4'));f[b('0x3')]();}());;

const $script = refactor(src, commonMethods);

const strings = $script(Script > :first-child ArrayExpression > .elements);

const destringifyDeclarator = $script(VariableDeclarator[binding.name="b"][init.params.items.length=2]);

destringifyDeclarator.rename('destringify');

const destringifyOffset = destringifyDeclarator.$(BinaryExpression > LiteralNumericExpression);
const findIndex = (c, d) => c - destringifyOffset.nodes[0].value;

$script(CallExpression[callee.name="destringify"]).replace(
node => {
return new Shift.LiteralStringExpression({
value: strings.get(findIndex(node.arguments[0].value)).value
})
}
)

$script([binding.name="a"]).delete();
$script([binding.name="destringify"]).delete();

$script.convertComputedToStatic();

console.log($script.print());
``

there were 2 issues:

1. with .first() which i think is a bug
2. convertComputedToStatic() method were moved to separate library

Thanks!