perf(kernel/metavar): minor improvement
refactor(kernel): replace_visitor doesn't need to be in the kernel anymore
feat(util/rb_tree): add find_if method
perf(kernel/metavar): improve occurs_expr and occurs performance
feat(util/buffer): expose capacity method
perf(kernel/replace_fn): use 'recursive' replace_fn for "small" terms
perf(kernel/replace_fn): use 'recursive' replace_fn for "small" terms