Fix gitlab issue #5.
The parse_float() function has several bugs:
1. For 1- and 2-word floats it will always write all 4 bytes if the value is
exactly 0.
2. It incorrectly rounds and normalizes 1- and 2-word floats.
To see the issue easily try the following inputs:
.flt2 1.0
.flt4 1.0
These will assemble as '040100 000000' and '040200 000000 000000 000000'.
They should both begin '040200'.
In fact the test file test/test-prec.mac is incorrect in its only floating point value:
.word ^f 1.5 ; 040140
should actually assemble as 040300, not 040140 (040140 is actually ^F0.875).
I confirmed this on RT-11 running MACRO V05.06.
I fixed the problem with the following deltas:
[the patch]
The most crucial change is the very last one. 0x200000000000 is actually (1 << 45)
and because ufrac is normalized it means that it will always downshift ufrac by 1.
