Add invokelatest barrier to string(...) in @assert

This change protects `@assert` from invalidations to `Base.string(...)`
which are fairly easy to trigger.

Perhaps the worst offender right now is `using StyledStrings`
which invalidates `string(::Expr)`, which unfortunately leads to
invalidation of almost `@assert`s (without an explicit message)

base/error.jl

@@ -232,12 +232,12 @@ macro assert(ex, msgs...)
         msg = msg # pass-through
     elseif !isempty(msgs) && (isa(msg, Expr) || isa(msg, Symbol))
         # message is an expression needing evaluating
-        msg = :(Main.Base.string($(esc(msg))))
+        msg = :(Main.Base.invokelatest(Main.Base.string, $(esc(msg))))
     elseif isdefined(Main, :Base) && isdefined(Main.Base, :string) && applicable(Main.Base.string, msg)
         msg = Main.Base.string(msg)
     else
         # string() might not be defined during bootstrap
-        msg = :(_assert_tostring($(Expr(:quote,msg))))
+        msg = :(Main.Base.invokelatest(_assert_tostring, $(Expr(:quote,msg))))
     end
     return :($(esc(ex)) ? $(nothing) : throw(AssertionError($msg)))
 end