Remove 0.12 Deprecations

[diesalbla]

WE remove all the items that have been marked as deprecated since versions 0.12.0 and 0.12.3, from September and October 2019 resp. This include the following elements:

• The CursorOp move operations to move N items to the right or left, (in a Json array) or to move the the first item in the array.
• The ACursor methods to do those same operations.
• The pretty method for printing.
• The AccumulatingDecoder class.

[travisbrown]

Looks good to me! Will merge when green.

[codecov-io]

Codecov Report

Merging #1683 (00d8af4) into master (8d66dd7) will increase coverage by 0.78%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1683      +/-   ##
==========================================
+ Coverage   86.17%   86.95%   +0.78%     
==========================================
  Files          61       59       -2     
  Lines        2423     2392      -31     
  Branches      130      134       +4     
==========================================
- Hits         2088     2080       -8     
+ Misses        335      312      -23     

Impacted Files	Coverage Δ
.../core/shared/src/main/scala/io/circe/ACursor.scala	71.42% <ø> (+6.91%)	⬆️
...core/shared/src/main/scala/io/circe/CursorOp.scala	55.55% <ø> (+2.43%)	⬆️
.../core/shared/src/main/scala/io/circe/Decoder.scala	86.13% <ø> (+0.49%)	⬆️
.../shared/src/main/scala/io/circe/FailedCursor.scala	72.72% <ø> (ø)
.../core/shared/src/main/scala/io/circe/HCursor.scala	90.62% <ø> (+18.12%)	⬆️
...les/core/shared/src/main/scala/io/circe/Json.scala	82.02% <ø> (+0.45%)	⬆️
.../core/shared/src/main/scala/io/circe/Printer.scala	95.21% <ø> (+0.87%)	⬆️
...d/src/main/scala/io/circe/cursor/ArrayCursor.scala	71.42% <ø> (-1.91%)	⬇️
.../src/main/scala/io/circe/cursor/ObjectCursor.scala	100.00% <ø> (ø)
...red/src/main/scala/io/circe/cursor/TopCursor.scala	100.00% <ø> (ø)
... and 4 more

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[travisbrown]

Looks good to me, thanks!