address a few compiler warnings

[fernomac]

Description of changes:

Hey friends! I was testing out the setup of a new laptop by giving this a build and figured I'd fix a couple quick compiler warnings while I was in the neighborhood:

• Allocating an empty slice and then appending to it -> directly allocating the resulting slice
• switch in.(type) -> switch in := in.(type) to avoid a second typecheck/cast in the case statements
• casting an int to a string is discouraged because what it actually does is string([]rune{rune(int)), but it looks like it's doing strconv.Itoa(int) (we had one of each :)).

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #175 (e80b07b) into master (84f0098) will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #175      +/-   ##
==========================================
- Coverage   74.68%   74.67%   -0.02%     
==========================================
  Files          25       25              
  Lines        4930     4928       -2     
==========================================
- Hits         3682     3680       -2     
  Misses        743      743              
  Partials      505      505              

Impacted Files	Coverage Δ
ion/skipper.go	60.37% <0.00%> (ø)
ion/binarywriter.go	75.59% <100.00%> (-0.17%)	⬇️
ion/marshal.go	71.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 84f0098...e80b07b. Read the comment docs.

[jobarr-amzn]

Looks good to me :)