refactor(ivy): flatten css selectors

[kara]

This PR flattens the CSS selectors generated for directives and components. This should help directives to generate less code, and will make selectors generally friendlier to work with.

Simple selector:
selector: 'div'

Parsed (before):

selector: [[['div'], null]]

Parsed (after):

selectors: [['div']]

Complex selector:
selector: 'div.foo[title=title]:not([bar]):not(.baz), [test]'

Parsed (before):

selector:  [
  [['div', 'title', 'title', 'class', 'foo'], [[ '', 'bar', '' ], ['', 'class', 'baz']]] , 
  [['', 'test', ''], null] 
]

Parsed (after, with flags written out for clarity):

selectors: [
   [ 'div', 'title', 'title',  SelectorFlags.CLASS, 'foo', 
      SelectorFlags.NOT|SelectorFlags.ATTRIBUTE, 'bar', 
      SelectorFlags.NOT|SelectorFlags.CLASS, 'baz'], 
   ['', 'test', '']
]

Because SelectorFlags is a const enum, the real output will be a number (see below):

Parsed (after, actual):

selectors: [
   [ 'div', 'title', 'title',  8, 'foo', 3, 'bar', 9, 'baz'], 
   ['', 'test', '']
]

Notes:

• I also changed selector to selectors in DirectiveDef because I think it clarifies that it is a list of selectors for the directive, as opposed to one selector.

First commit: updates runtime
Second commit: updates compiler

[mhevery]

I think this should be number => SelectorFlags?

[kara]

presubmit

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