operand: fix integer float data

[mmcloughlin]

Issue #387 pointed out that integer float data is printed incorrectly, such
that it is not parsed correctly by the Go assembler. Specifically, integer
values need the decimal point, otherwise they will be treated as integers. For
example, 1 must be represented as $(1.) or $(1.0) to be parsed correctly.

This PR fixes that problem and adds a regression test. The root of the
problem was that the formatting verb %#v does not have the right behavior
for integers. We fix it by deferring to custom String() function for the
float operand types.

Fixes #387
Closes #388

[codecov-commenter]

Codecov Report

Merging #393 (595ff46) into master (9bef88d) will increase coverage by 0.04%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master     #393      +/-   ##
==========================================
+ Coverage   78.12%   78.16%   +0.04%     
==========================================
  Files          99      100       +1     
  Lines       23728    23768      +40     
==========================================
+ Hits        18537    18578      +41     
+ Misses       5095     5094       -1     
  Partials       96       96              

Flag	Coverage Δ
integration	22.20% <100.00%> (+0.15%)	⬆️
stress	?
unittests	73.20% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
operand/const.go	95.00% <100.00%> (+3.33%)	⬆️
operand/zconst.go	66.66% <100.00%> (ø)
tests/fixedbugs/issue387/asm.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes