Remove mutable render state from Liquid::Template

Use Liquid::Context to access this mutable state after rendering or to have
state that is persisted between mutations

README.md

@@ -80,22 +80,24 @@ It is also recommended that you use it in the template editors of existing apps
 
 By default, the renderer doesn't raise or in any other way notify you if some variables or filters are missing, i.e. not passed to the `render` method.
 You can improve this situation by passing `strict_variables: true` and/or `strict_filters: true` options to the `render` method.
-When one of these options is set to true, all errors about undefined variables and undefined filters will be stored in `errors` array of a `Liquid::Template` instance.
+When one of these options is set to true, all errors about undefined variables and undefined filters will be stored in an `errors` array on the `Liquid::Context` instance used for rendering.
 Here are some examples:
 
 ```ruby
 template = Liquid::Template.parse("{{x}} {{y}} {{z.a}} {{z.b}}")
-template.render({ 'x' => 1, 'z' => { 'a' => 2 } }, { strict_variables: true })
+context = Liquid::Context.new({ 'x' => 1, 'z' => { 'a' => 2 } })
+template.render(context, { strict_variables: true })
 #=> '1  2 ' # when a variable is undefined, it's rendered as nil
-template.errors
+context.errors
 #=> [#<Liquid::UndefinedVariable: Liquid error: undefined variable y>, #<Liquid::UndefinedVariable: Liquid error: undefined variable b>]
 ```
 
 ```ruby
 template = Liquid::Template.parse("{{x | filter1 | upcase}}")
-template.render({ 'x' => 'foo' }, { strict_filters: true })
+context = Liquid::Context.new({ 'x' => 'foo' })
+template.render(context, { strict_filters: true })
 #=> '' # when at least one filter in the filter chain is undefined, a whole expression is rendered as nil
-template.errors
+context.errors
 #=> [#<Liquid::UndefinedFilter: Liquid error: undefined filter filter1>]
 ```
 
@@ -111,4 +113,4 @@ template.render!({ 'x' => 1}, { strict_variables: true })
 
 To help track usages of a feature or code path in production, we have released opt-in usage tracking. To enable this, we provide an empty `Liquid:: Usage.increment` method which you can customize to your needs. The feature is well suited to https://github.com/Shopify/statsd-instrument. However, the choice of implementation is up to you.
 
-Once you have enabled usage tracking, we recommend reporting any events through Github Issues that your system may be logging. It is highly likely this event has been added to consider deprecating or improving code specific to this event, so please raise any concerns.
+Once you have enabled usage tracking, we recommend reporting any events through Github Issues that your system may be logging. It is highly likely this event has been added to consider deprecating or improving code specific to this event, so please raise any concerns.