This change fixes two distinct data races:
1) some global vars of type *itype of the interp package are actually
mutated during the lifecycle of an Interpreter. Even worse: if more than
one Interpreter instance are created and used at a given time, they are
actually racing each other for these global vars.
Therefore, this change replaces these global vars with generator
functions that create the needed type on the fly.
2) the symbols given as argument of Interpreter.Use were directly copied
as reference (since they're maps) when mapped inside an Interpreter
instance. Since the usual case is to give the symbols from the stdlib
package, it means when the interpreter mutates its own symbols in
fixStdio, it would actually mutate the corresponding global vars of the
stdlib package. Again, this is at least racy as soon as several
instances of an Intepreter are concurrently running.
This change fixes the race by making sure Interpreter.Use actually
copies the symbol values instead of copying the references.
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