to_big(tostring(x)) does not equal x, unexpectedly

[english5040]

(using Omeganum, have not checked the other implementation)

If x is a bignum 1000, to_string(x) = '1,000' (with a comma). bignums use number_format to print themselves.
'1,000' then fails to parse.

Since this behavior is different from plain Lua, this issue is prone to cause bugs.