Strict assignment

[k4b7]

TODO:

• strict assignment for variable declaration/assignment
• strict assignment for function calls
• check that overloaded function calls work as expected
• error and option messages
• enable strict assignment for JSX
• update baselines for tsc --init (tsconfig.json) tests
• add unit tests for the new behavior
• add a test to check deeply nested types
• handle callback params correctly
• allow covariance for object types with simple properties
• disallow assignment of readonly types to the mutable equivalent

[k4b7]

Removing this line results in code in test.ts using these declarations with strict-assignment checking even if when using the --strict-assignment flag.