Begin working on macro expansion. So far this just expands backquotes.

Also some changes to SyntaxNode make it possible to construct ASTs from
these programmatically, without the source being available. And some
basic support to allow us to track provenance through syntax
transformations.

Allow `macroexpand` to call macros which only have an implementation in
terms of the `Expr` data structure.

Also add a `__macroname__` argument to pass the expression of the macro
name as it appeared at macro invocation. Implement `@__FILE__`
`@__LINE__` and `@__COLUMN__` in terms of this.

Macros expansion of macros which emit macros as part of their AST needs
care when that AST contains interpolations: We don't want the second
macro expansion to see the hygenic_scope expression. One way to do this
is to transform hygenic_scope back into `SyntaxLiteral` on the fly.

Also fix auto-escaping for expressions arising from the original scope
by adding the necessary number of Expr(:esc)

One of the many internal Base things we're using is likely different on
<= 1.8

Plus subtle fixes to macro expansion prototype