Refactor active configuration handling

[haxscramper]

Refactor active configuration handling in the options.ConfigRef - right now every single field is placed on the same level, including both mutable non-configuration values (error counter, list of symbols) and active/passive configurations (list of active notes, local and global options).

Field that are used to store explicit configuration values (backend/target/options/globalOptions) need to be factored out into a separate type definition.

• This would allow treating compiler (at least on conceptual level) as a function proc compile(config: ActiveOptions).
• This type can be reused in testament to serve as a configuration cell in test matrix
• Command-line configuration reading can be simplified as well - instead of using a whole ConfigRef object and mutating it in-place, it can simply return ActiveOptions object

Old code should continue to work using getters and setters. Additional upside - this change would provide a single point of tracing for the active configuration and reduce number of direct access to fields (which cannot be debugged with echo, cannot be asserted and so on).

type
+ ActiveOptions = object
+   options*: TOptions

  ConfigRef* {.acyclic.} = ref object 
-   options*: TOptions
+   active: ActiveOptions

+ func options*(conf: ConfigRef): TOptions = conf.active.options
+ proc `options=`*(conf: ConfigRef, opts: TOptions) = conf.active.options = opts

Repeated for all "input" fields: backend, target, options, filenamesOption