New patch implementation and DSL

This commit introduces a new patch implementation that supports
checksums and caching.

Patches are declared in blocks:

  patch do
    url ...
    sha1 ...
  end

A strip level of -p1 is assumed. It can be overridden using a symbol
argument:

  patch :p0 do
    url ...
    sha1 ...
  end

Patches can be declared in stable, devel, and head blocks. This form is
preferred over using conditionals.

  stable do
    # ...

    patch do
      url ...
      sha1 ...
    end
  end

Embedded (__END__) patches are declared like so:

  patch :DATA
  patch :p0, :DATA

Patches can also be embedded by passing a string. This makes it possible
to provide multiple embedded patches while making only some of them
conditional.

  patch :p0, "..."

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