automatically defined in the preamble. Note that there is no way for C
code to create a value of this type; this is only useful for passing
string values from Go to C and back to Go.

Using //export in a file places a restriction on the preamble:
since it is copied into two different C output files, it must not
contain any definitions, only declarations. If a file contains both

// A TokenReader is like a reader, but returns lex tokens of type Token. It also can tell you what
// the text of the most recently returned token is, and where it was found.
// The underlying scanner elides all spaces except newline, so the input looks like a stream of
// Tokens; original spacing is lost but we don't need it.
type TokenReader interface {
	// Next returns the next token.
	Next() ScanToken