Provide the LLVM as a compiler backend

[drhagen]

This uses llvmlite to build LLVM code from the Tensora IR. Use of the LLVM backend is controlled with the tensora.compile.BackendCompiler.{llvm,cffi} flag to TensorMethod. This is also exposed as the -l llvm option to the CLI, which prints out LLVM IR.

llvmlite has a ctypes interface, but this PR still uses cffi for the taco tensor structures and invoking the compiled function.

Implements #66.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 83.64389% with 79 lines in your changes missing coverage. Please review.

Project coverage is 90.53%. Comparing base (bc2013a) to head (720de80).

Files	Patch %	Lines
src/tensora/codegen/_ir_to_llvm.py	80.45%	45 Missing and 6 partials ⚠️
src/tensora/compile/_porcelain.py	78.72%	8 Missing and 2 partials ⚠️
src/tensora/compile/_tensor_method.py	82.92%	2 Missing and 5 partials ⚠️
src/tensora/generate/_base.py	78.57%	3 Missing and 3 partials ⚠️
src/tensora/codegen/_type_to_llvm.py	91.17%	3 Missing ⚠️
src/tensora/ir/ast.py	71.42%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
- Coverage   92.72%   90.53%   -2.19%     
==========================================
  Files          53       60       +7     
  Lines        2761     3150     +389     
  Branches      498      540      +42     
==========================================
+ Hits         2560     2852     +292     
- Misses        125      206      +81     
- Partials       76       92      +16     

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