Add saturating float truncation operators (#573)

Add saturating truncation operators as described here:
https://github.com/webassembly/nontrapping-float-to-int-conversions.

This change also codifies the mechanism for enabling new WebAssembly features
by passing the `Features` object. Opcode now has a `IsEnabled(const Features&)`
member function to query if the opcode is enabled. This means that the
`--future-exceptions` flag has been renamed to `--enable-exceptions` for
consistency. Checking whether the feature is enabled always happens at input;
either WastParser or BinaryReader.

0 files changed, 0 insertions, 0 deletions