Partial support for destructuring in block binding

This handles patterns in let and const declarations but not yet
in loop headers.

This also simplifies the generated code, in that it does not
hoist a variable declaration when it is not needed.

Issue #1276
0.0.66
Pass metadata to REPL onFailure.

Show transcoding if the error is an evaluation error.

Fixes #1373

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1375

Closes #1375, closes #1373.
Use Compiler API in Loader.

Now our command line always runs through our Compiler API.

Fixes #1324

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1355

Closes #1355, closes #1324.
Block bindings: Fix issue with sibling scopes

This uses the free variable checker to see if there is a free
variable with the same name in the current variable scope.

Fixes #1358
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