Improve rhotacization mark

[brawer]

Some phonetic texts and dictionaries use ˞ U+02DE modifier letter rhotic hook. The modifier mostly comes after e. In Source Code Pro, however, the string ə˞ (or the larger wə˞ dɪsˈpjutɪŋ) doesn’t look so great:

Compare to Apple’s Menlo font, which makes ə˞ two ems wide:

[pauldhunt]

some very good catches, thank you. in the future do you mind grouping all of these similar issues together? Thanks.