Fixed point converter improvements; FpxxConverter pipelining

[fayalalebrun]

This adds several improvements to the converters from and to fixed point:

• Fixed point to floating point now supports fractional bits
• Uses AFix and Flow in I/O
• Uses pipelining API
• Can optionally generate and consume flags to preserve the status of inf/nan

Additionally, I pipelined FpxxConverter according to the bottlenecks I observed in Vivado synthesis. However, depending on the source and output floating point format, the delay and resource consumption of each path may vary greatly. Therefore, instead of a number, this component takes a tuple of booleans corresponding to whether each pipeline stage is enabled.