Simplify Dependency Injection

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

• Removes the AddImageSharpCore extensions. We don't need them anymore and they're confusing.
• Removes weird registration of services via options sub properties.
• Adds ClearProviders and ClearProcessors to allow simplified reset of registered services.
• Use ArrayPool for supplying buffers for hashing long strings.
• Fix missing .Web suffix from readme.

[codecov]

Codecov Report

Merging #115 into master will increase coverage by 0.01%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
+ Coverage   84.01%   84.03%   +0.01%     
==========================================
  Files          47       47              
  Lines        1126     1121       -5     
  Branches      161      162       +1     
==========================================
- Hits          946      942       -4     
  Misses        128      128              
+ Partials       52       51       -1     

Flag	Coverage Δ
#unittests	84.03% <94.11%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...DependencyInjection/ServiceCollectionExtensions.cs	94.11% <80.00%> (-1.34%)	⬇️
src/ImageSharp.Web/Caching/CacheHash.cs	100.00% <100.00%> (+23.52%)	⬆️
...DependencyInjection/ImageSharpBuilderExtensions.cs	69.64% <100.00%> (ø)
src/ImageSharp.Web/FormatUtilities.cs	100.00% <100.00%> (ø)

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