Updated Language Specification

[ahejlsberg]

Fixes #1176.
Fixes #1267.
Fixes #1626.
Fixes #1691.
Fixes #1770.
Fixes #1808.

This PR updates the language specification with the following:

• Updates to parameter declarations (3.8.2.2).
• Spread elements in array literals (4.6).
• Updates to contextually typed function expressions (4.9.3).
• Destructuring assignments (4.17.1).
• Updates to contextually typed expressions (4.19).
• Updates to type guards (4.20).
• Destructuring variable declarations (5.1.2).
• Destructuring parameter declarations (6.4).
• Expanded constant enum expressions (9.2).
• const enum declarations (9.4).
• Updates to ambient enum declarations (12.1.4).
• Correction to ambient external module grammar (12.2).