Implement block shapes to latest spec

-Convert most constants in the code to use multiplications of GRID_UNIT (4px). This lets us be consistent with Carl's specs and helps explain the numbers a little better.
-Implement notches using cubic curves instead of arcs. This version matches the spec pretty much exactly. As part of this, remove NOTCH_BASE_HEIGHT which is no longer relevant.
-Slight simplification of renderCompute_ (which calculates metrics).
-Fixed SEP_SPACE_X and SEP_SPACE_Y to be the correct number of grid units. Our blocks before were slightly too small, now they are the right size.
-Implement the correct corner radius, hat/end hat radius.
-Add fixed ICON_WIDTH and ICON_HEIGHT, i.e., always size the icons to 40x40px as according to the spec.
-Add fixed STATEMENT_BLOCK_SPACE to represent the size of the bar above forever/repeat/pants blocks. Previously the code was assuming this was SEP_SPACE_Y, when in fact the spec has it as larger (4 units).
-Add accurate MIN_BLOCK_X and MIN_BLOCK_Y for simpler calculation of minimum bay size.
-Remove some unused SVG paths (INNER_TOP_LEFT_CORNER, INNER_BOTTOM_LEFT_CORNER).
-Remove extra code in getHeightWidth which caused a bug in rendering forever-in-repeat.
-Fix stroke width around the blocks to be 1px, according to spec.

I've checked that everything lines up with Carl's spec images at 8x zoom.

This should implement #69, #70, #71, #72, #74, and #75 (not 73 as we haven't done that input yet).

core/css.js

@@ -179,8 +179,7 @@ Blockly.Css.CONTENT = [
   '}',
 
   '.blocklyPath {',
-    // 'stroke: #f00;',
-    'stroke-width: 1.5px;',
+    'stroke-width: 1px;',
   '}',
 
   '.blocklySelected>.blocklyPath {',