Pipe ceil, trunc and floor through round

[lgoettgens]

similar to how it is done in julia Base. Also make use of the generic methods from Base where possible, to avoid invalidations.
This PR reduces the number of invalidations of loading DecFP from 517 to 437 (on julia 1.12.5). In particular, the following invalidation trees get resolved:

 inserting floor(::Type{Int64}, x::Dec128) @ DecFP ~/code/julia/DecFP.jl/src/DecFP.jl:552 invalidated:
   backedges: 1: superseding floor(::Type{T}, x) where T @ Base rounding.jl:475 with MethodInstance for floor(::Type{Int64}, ::Any) (3 children)

 inserting ceil(::Type{Int64}, x::Dec128) @ DecFP ~/code/julia/DecFP.jl/src/DecFP.jl:553 invalidated:
   backedges: 1: superseding ceil(::Type{T}, x) where T @ Base rounding.jl:476 with MethodInstance for ceil(::Type{Int64}, ::Any) (30 children)

 inserting ceil(::Type{I}, x::DecFP.DecimalFloatingPoint) where I<:Integer @ DecFP ~/code/julia/DecFP.jl/src/DecFP.jl:576 invalidated:
   backedges: 1: superseding ceil(::Type{T}, x) where T @ Base rounding.jl:476 with MethodInstance for ceil(::Type{Integer}, ::Any) (59 children)

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.00%. Comparing base (a572ebd) to head (dae0967).

Files with missing lines	Patch %	Lines
src/DecFP.jl	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #200      +/-   ##
==========================================
- Coverage   96.12%   94.00%   -2.12%     
==========================================
  Files           2        2              
  Lines         593      584       -9     
==========================================
- Hits          570      549      -21     
- Misses         23       35      +12     

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