Simplify specifying block params

[dfreeman]

This change reworks how block signatures are specified, going from this:

declare const MyComponent: (args: NoNamedArgs) => AcceptsBlocks<{
  default(foo: string, bar: number): BlockResult;
  inverse?(): BlockResult;
}>;

...to this:

declare const MyComponent: (args: NoNamedArgs) => AcceptsBlocks<{
  default: [string, number];
  inverse?: [];
}>;

This eliminates the need for the useless BlockResult dummy type. It may appear to lose useful information in the form of parameter names, but in fact that already disappears as the type is transformed, and it will also be possible in TS 4 to label tuple elements.

It also (unfortunately) includes a refactoring to reorganize built-in template entities like let and each. Ideally I would have landed that separately, but by the time I realized how much of an impact these two changes would have individually they were already tangled up with one another.

Regardless, I think there's likely to be further refactoring of these things in the future, since the items available look fairly different between GlimmrX, Glimmer.js, and Ember.

[chriskrycho]

I appreciate the write up! Still following your work on this even though I haven’t been able to actively contribute.