config, src: enable nimPreviewSlimSystem

[ee7]

From the Nim 2.0.0 detailed changelog (emphasis added):

• Certain definitions from the default system module have been moved to the following new modules:

  • std/syncio
  • std/assertions
  • std/formatfloat
  • std/objectdollar
  • std/widestrs
  • std/typedthreads
  • std/sysatomics

  In the future, these definitions will be removed from the system module, and their respective modules will have to be imported to use them. Currently, to make these imports required, the -d:nimPreviewSlimSystem option may be used.

• Enabling -d:nimPreviewSlimSystem also removes the following deprecated symbols in the system module:

  • Aliases with an Error suffix to exception types that have a Defect suffix (see exceptions): ArithmeticError, DivByZeroError, OverflowError, AccessViolationError, AssertionError, OutOfMemError, IndexError, FieldError, RangeError, StackOverflowError, ReraiseError, ObjectAssignmentError, ObjectConversionError, FloatingPointError, FloatOverflowError, FloatUnderflowError, FloatInexactError, DeadThreadError, NilAccessError
  • addQuitProc, replaced by exitprocs.addExitProc
  • Legacy unsigned conversion operations: ze, ze64, toU8, toU16, toU32
  • TaintedString, formerly a distinct alias to string
  • PInt32, PInt64, PFloat32, PFloat64, aliases to ptr int32, ptr int64, ptr float32, ptr float64

• Enabling -d:nimPreviewSlimSystem removes the import of channels_builtin in the system module, which is replaced by threading/channels. Use the command nimble install threading and import threading/channels.

Closes: #760

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To-do:

• [ ] Add the remaining required imports.

This PR is currently blocked by (at least) jsony not supporting this.