Remove dead code and fix documentation.

[johanneskloos]

The documentation for BatInt.of_string was misleading:
The current implementation throws Failure, not Invalid_argument.

This is because the implementation being referred to was dead code,
which is being removed in this commit.

[johanneskloos]

This patch makes BatInt compatible with BatFloat, BatInt32 and BatInt64. If you prefer, I also have a patch that works the other way round, changing the exception behviour of {Batint,BatFloat,BatIn32,BatInt64}.of_string to raise Invalid_argument instead of failure.

[gasche]

Using Failure is the correct choice to remain consistent with the compiler distribution's standard library: int_of_string, or Int32.of_string, both use Failure rather than Invalid_argument. Thanks!