prevent bindings interfering with each other on the same element

[Rich-Harris]

Fixes a bug with audio bindings — when replaying an ended audio track, it should return to the start, but because each of currentTime, duration and paused had their own 'lock' variables, state was getting out of sync with the DOM. Fixed by sharing a lock variable between all bindings on the same element.

It does mean we have var audio_updating = false repeated once per binding. I think the solution to that would be to add a block.addVariable method that declares all (or most) variables in a single block at the top — this would also make the code marginally easier to read (in my opinion):

// before
var p = createElement( 'p' );
var text_value = root.foo;
var text = createText( text_value );
appendNode( text, p );
var text_1 = createText( "\n" );
var p_1 = createElement( 'p' );
var text_2_value = root.bar;
var text_2 = createText( text_2_value );
appendNode( text_2, p_1 );
var text_3 = createText( "\n" );
var p_2 = createElement( 'p' );
var text_4_value = root.baz;
var text_4 = createText( text_4_value );
appendNode( text_4, p_2 );

// after
var text_value, text_2_value, text_4_value;

var p = createElement( 'p' );
var text = createText( text_value = root.foo );
appendNode( text, p );
var text_1 = createText( "\n" );
var p_1 = createElement( 'p' );
var text_2 = createText( text_2_value = root.bar );
appendNode( text_2, p_1 );
var text_3 = createText( "\n" );
var p_2 = createElement( 'p' );
var text_4 = createText( text_4_value = root.baz );
appendNode( text_4, p_2 );