Improved `glob()` implementation; filsystem-prewarm

Most important: glob() works more accurately now. Previously **/foo.txt was not matching the plain foo.txt while now **/ is correctly interpreted as zero or more directory elements. Also, the matching was improved to only delegate to regular expressions if needed, reducing expensive expressions.

Support to get the most out of slow file-systems has been added: on slow network filesystems or cold-storage CI machines it might make sense to enable it by creating a ~/.cache/bant/ directory that bant then makes use of for this feature.