Correct floating point trunc, floor, ceil

[TotalVerb]

@omus, after a little thought (and with help from @musm) I've decided that this is a more complete fix for the inaccurate floats. What this is doing behind the scenes is using double double arithmetic to work around the loss of precision from the multiplication.

I've modified the tests slightly but they should be a superset of the ones that you added (Thanks for that!).

[coveralls]

Coverage decreased (-0.2%) to 96.209% when pulling d665598 on fw/accurate-trunc into 0e56bea on master.

[coveralls]

Coverage decreased (-0.2%) to 96.209% when pulling d665598 on fw/accurate-trunc into 0e56bea on master.

[coveralls]

Coverage decreased (-0.2%) to 96.209% when pulling d665598 on fw/accurate-trunc into 0e56bea on master.

[coveralls]

Coverage decreased (-0.2%) to 96.209% when pulling d665598 on fw/accurate-trunc into 0e56bea on master.

[coveralls]

Coverage decreased (-0.2%) to 96.209% when pulling d665598 on fw/accurate-trunc into 0e56bea on master.

[coveralls]

Coverage decreased (-0.2%) to 96.209% when pulling d665598 on fw/accurate-trunc into 0e56bea on master.

[codecov]

Codecov Report

Merging #83 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
+ Coverage    96.4%   96.44%   +0.04%     
==========================================
  Files          17       17              
  Lines         417      422       +5     
==========================================
+ Hits          402      407       +5     
  Misses         15       15

Impacted Files	Coverage Δ
src/FixedPointDecimals/FixedPointDecimals.jl	100% <100%> (ø)	⬆️

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[coveralls]

Coverage decreased (-0.2%) to 96.209% when pulling f0bca22 on fw/accurate-trunc into 0e56bea on master.

[coveralls]

Coverage decreased (-0.2%) to 96.209% when pulling f0bca22 on fw/accurate-trunc into 0e56bea on master.

[coveralls]

Coverage decreased (-0.2%) to 96.209% when pulling f0bca22 on fw/accurate-trunc into 0e56bea on master.

[coveralls]

Coverage increased (+0.04%) to 96.445% when pulling f976eba on fw/accurate-trunc into 0e56bea on master.

[coveralls]

Coverage increased (+0.04%) to 96.445% when pulling f976eba on fw/accurate-trunc into 0e56bea on master.

[coveralls]

Coverage increased (+0.04%) to 96.445% when pulling f976eba on fw/accurate-trunc into 0e56bea on master.

[coveralls]

Coverage increased (+0.04%) to 96.445% when pulling f976eba on fw/accurate-trunc into 0e56bea on master.

[coveralls]

Coverage increased (+0.04%) to 96.445% when pulling f976eba on fw/accurate-trunc into 0e56bea on master.