Update some floating-point syntax files.

Improve floating-point rounding conversions.

The conversions should now work for more float types, e.g. evaluation for "(23, 8 + 1) float" was not working before.

Update floating-point theory to specify exception flags for some operations.

• The flags are: DivideByZero, InvalidOp, Overflow, Precision and Underflow.
• The arithmetic operations now return a pair (flags and result).
• Native (oracle) evaluation has been overhauled. It now only applies to some 64-bit machine_ieeeTheory operations directly.
• The L3 Importer has been updated.

Floating-point update.

• isSignallingNan predicate is added to machine_ieeeTheory.
• The RISC-V model is simplified in places (for DIV and REM).
• The L3 import is extended to cover more operations.

Remove float_negate1985 and float_abs1985

This seems to have been half done in
086cbeb857ffbcc88784f4f81db7626771ad8758

Minor changes within floating-point theory.

• The operations float_negate1985 and float_abs1985 have been removed.
• Added a predicate float_is_signalling and renamed float_some_nan to float_some_qnan.
• Fixed problem with EQ_tm and GT_tm being swapped in binary_ieeeSyntax.

Improve handling of NaNs in binary_ieeeTheory.

The choice of NaN is now a function of the operation being performed. Previously all instances of float_some_nan would be the same (for any given float type).

Ensure most of the "floating-point" development works with Moscow ML.

The computeLib support still relies on some SML structures that aren't yet available under Moscow ML (2.10).

Add "fused multiply and subtract" floating-point operation

Update for the latest version of L3, which has additional floating-point support.

Cover IEEE754:1985 behaviour for "abs" and "neg". Also adds support for "compare", "toInt" and "fromInt" at the machine level.

Relocate examples/l3-machine-code/common/fp to src/floating-point.