Commits on Sep 20, 2019

1. hdl.mem: use 1 as reset value for ReadPort.en. …

   This is necessary for consistency, since for transparent read ports,
   we currently do not support .en at all (it is fixed at 1) due to
   YosysHQ/yosys#760. Before this commit, changing port transparency
   would require adding or removing an assignment to .en, which is
   confusing and error-prone.
   
   Also, most read ports are always enabled, so this behavior is also
   convenient.

   whitequark committed Sep 20, 2019
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2. lib.fifo: work around Yosys issue with handling of \TRANSPARENT. …

   Because of YosysHQ/yosys#1390, using a transparent port in AsyncFIFO,
   instead of being a no-op (as the semantics of \TRANSPARENT would
   require it to be in this case), results in a failure to infer BRAM.
   
   This can be easily avoided by using a non-transparent port instead,
   which produces the desirable result with Yosys. It does not affect
   the semantics on Xilinx platforms, since the interaction between
   the two ports in case of address collision is undefined in either
   transparent (WRITE_FIRST) or non-transparent (READ_FIRST) case, and
   the data out of the write port is not used at all.
   
   Fixes #172.

   whitequark committed Sep 20, 2019
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3. lib.fifo: fix doc typo. NFC.

   whitequark committed Sep 20, 2019
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