The interpreter has its own internal fileset and expects all code to have been parsed using that fileset. If a user creates a fileset, calls `go/parser.Parse*`, then passes the result to `interp.CompileAST`, strange things can happen.
The solutions I can see are:
1. Expose the fileset so the user can use it when parsing source.
2. Add the fileset as an option (input to New) so that the user can tell the interpreter to use a specific fileset.
3. Pass the fileset into `CompileAST`
There are two ways to implement option 3. One is to add a field to nodes and update every reference to `interp.fset` to use `node.fset`. The other is to add a parameter to every function that references `interp.fset` or calls a function that does. Both of these are significant changes and involve an extra pointer for every node or most function calls.
Options 1 and 2 are easy. Option 2 involves adding an option so I went with option 1. I can imagine situations where option 2 could be necessary, but I can open another issue/PR if and when I need that.
Fixes#1383
Adds CompileAST, which can be used to compile Go AST directly. This
allows users to delegate parsing of source to their own code instead of
relying on the interpreter.
CLoses#1251
