Remove the use of dmlc object registry

[hcho3]

Extracted from #285.

• Do not use the header dmlc/registry.h.
• Do not use the object registry provided by dmlc-core. The registry lets you instantiate a new object by the class name. We really don't need the registry, since there are only two kinds of code generators (FailSafeCompiler and ASTNativeCompiler). Simple if / else statements suffice.
• Use pimpl pattern to hide templated functions in the code generators.
• Define new headers failsafe.h and ast_native.h to store the class declarations of FailSafeCompiler and ASTNativeCompiler. This is more in line with conventional C++.
• Rename param to param_.

[codecov]

Codecov Report

Merging #288 (2eb3565) into mainline (262f782) will increase coverage by 0.02%.
The diff coverage is 77.33%.

@@              Coverage Diff               @@
##             mainline     #288      +/-   ##
==============================================
+ Coverage       84.99%   85.02%   +0.02%     
  Complexity         46       46              
==============================================
  Files             103      105       +2     
  Lines            7959     7958       -1     
  Branches           50       50              
==============================================
+ Hits             6765     6766       +1     
+ Misses           1169     1167       -2     
  Partials           25       25              

Impacted Files	Coverage Δ
include/treelite/compiler.h	100.00% <ø> (ø)
src/compiler/compiler.cc	96.22% <66.66%> (-0.14%)	⬇️
src/compiler/failsafe.cc	87.90% <66.66%> (+0.46%)	⬆️
src/compiler/ast_native.cc	96.34% <90.32%> (+0.20%)	⬆️
src/compiler/ast_native.h	100.00% <100.00%> (ø)
src/compiler/failsafe.h	100.00% <100.00%> (ø)

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