Delay assignment of node ids until after expansion. Ensures that each AST node
has a unique id. Fixes numerous bugs in macro expansion and deriving. Add two
representative tests.

Fixes #7971
Fixes #6304
Fixes #8367
Fixes #8754
Fixes #8852
Fixes #2543
Fixes #7654
auto merge of #9088 : nikomatsakis/rust/issue-6304-AST-tree-not-DAG, r=catamorphism

Ensures that each AST node has a unique id. Fixes numerous bugs in macro expansion and deriving. Add two
representative tests.

Fixes #7971
Fixes #6304
Fixes #8367
Fixes #8754
Fixes #8852
Fixes #2543
Fixes #7654