stacktraces for macroexpand

[vtjnash]

This PR allows errors thrown in macroexpand to propagate out directly, rather than discarding their stacktrace in order to wrap them in Expr(:error) constructs. It also makes the flisp-julia interface non-reentrant, allowing us to eliminate some complexity from ast.c.

Obligatory examples:

julia> macro error(msg); error(msg); end
@error (macro with 1 method)

julia> macroexpand(Main, :(@error "in a macro"))
ERROR: LoadError: in a macro
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] @error(::LineNumberNode, ::Module, ::Any) at ./REPL[1]:1
 [3] #macroexpand#39 at ./expr.jl:84 [inlined]
 [4] macroexpand(::Module, ::Any) at ./expr.jl:83
while loading REPL[2], in expression starting on line 1

julia> @printf "%z" b
ERROR: LoadError: ArgumentError: invalid printf format string: "%z"
Stacktrace:
 [1] next_or_die(::String, ::Int64) at ./printf.jl:85
 [2] parse1 at ./printf.jl:126 [inlined]
 [3] parse(::String) at ./printf.jl:46
 [4] gen(::String) at ./printf.jl:14
 [5] _printf(::String, ::Symbol, ::String, ::Tuple{Symbol}) at ./printf.jl:1148
 [6] @printf(::LineNumberNode, ::Module, ::Vararg{Any,N} where N) at ./printf.jl:1218
while loading REPL[8], in expression starting on line 1