fmgr interface mopup work. Use new DatumGetBool and BoolGetDatum

fmgr interface mopup work.  Use new DatumGetBool and BoolGetDatum
macros where appropriate (the code used to have several different ways
of doing that, including Int32, Int8, UInt8, ...).  Remove last few
references to float32 and float64 typedefs --- it's all float4/float8
now.  The typedefs themselves should probably stay in c.h for a release
or two, though, to avoid breaking user-written C functions.