refactor(util): rename fixed_size_allocator to memory_pool

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
perf(util/memory_pool): use memory_pool for hierarchical names and justification objects we get a 8% performance improvement when using multiple threads

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(*): rename Bool to Prop

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/standard): rename bit to bool

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/standard): rename bool_decidable to prop_decidable

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/standard/unit): make unit type similar to the one in the hott library

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