feat(frontends/lean): use 'begin-end' instead of 'proof-qed' for blocks of tactics, closes #81

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/unifier): we should preprocess choice constraints before we start solving any constraint, fixes #85

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/server): crash: uninitialized memory

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/parser): missing identifier information, fixes #83

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