Interpreter::run() was so far being used both as the "public API entry
point" for running a JS::Program as well as internally to execute
JS::Statement|s of all kinds - this is now more distinctly separated.
A program as returned by the parser is still going through run(), which
is responsible for creating the initial global call frame, but all other
statements are executed via execute_statement() directly.
Fixes#3437, a regression introduced by adding ASSERT(!exception()) to
run() without considering the effects that would have on internal usage.
