chore(testcov): add compile target "cov" to run code-coverage locally

 - need to run cmake with "-DTESTCOV=ON" and "-DCMAKE_BUILD_TYPE=Debug"
 - type "make/ninja cov"
 - open "coverage/index.html" to check the code coverage
feat(splay_map): add operator<< to splay_map for debugging
doc(coverage): update instructions for code coverage

[skip ci]
chore(testcov): add missing CodeCoverage.cmake
fix(testcov): install lcov and include testcov only when it's on
feat(scoped_map): add operator<< to scoped_map for debugging
refactor(fo_match): use scoped_map
refactor(rewrite): use scoped_map as a type of substitution
test(fo_match): add more unittests

[skip ci]
doc(coverage): add instructions for lcov-1.10

[skip-ci]
refactor(tests/frontends/lean): use consistent name convention for file names

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(lean/scanner): add more tests for improving coverage

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(lean/parser): add more tests for improving coverage

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>