fix(bin/ltags): open .ilean files with UTF-8 encoding

fix #376
refactor(library/logic/connectives): rename theorems
refactor(library/logic): rename theorems
feat(library/init): create markdown directory file
refactor(library): rename exists_elim and exists_intro to exists.elim
and exists.intro
refactor(library): rename theorems "iff.flip_sign -> not_iff_not_of_iff" and "decidable_iff_equiv -> decidable_of_decidable_of_iff"
feat(init): reserve notation for "not in"
fix(library/normalize): bug in the "eval" command
refactor(library): add compile_equations function, generic_exception, and cleanup elaborator_exception
test(tests/lean/extra): more tests for equations compiler
checkpoint
checkpoint