Add `pledgeinsize`

[nhz2]

This is part of https://github.com/JuliaIO/TranscodingStreams.jl/pull/239

This PR reduces the allocations needed when decompressing data compressed with transcode.

For example:

julia> using BenchmarkTools

julia> using CodecZstd

julia> c = transcode(ZstdCompressor, rand(UInt8, 10000));

julia> @btime transcode($(ZstdDecompressor()), $(c));

With this PR and https://github.com/JuliaIO/TranscodingStreams.jl/pull/239: 416.579 ns (4 allocations: 10.05 KiB)

Before: 510.451 ns (5 allocations: 19.72 KiB)

[codecov[bot]]

Codecov Report

:x: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 63.11%. Comparing base (eef39ab) to head (db69f63). :warning: Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/libzstd.jl	66.66%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
+ Coverage   61.49%   63.11%   +1.61%     
==========================================
  Files           4        4              
  Lines         361      366       +5     
==========================================
+ Hits          222      231       +9     
+ Misses        139      135       -4     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[mkitti]

What is the status of this pull request at the moment?

[nhz2]

It has some minor conflicts that need to be resolved. I also want to try to maintain support for older versions of TranscodingStreams.