Code changes for `v0.3.0`

[kaipartmann]

A tracker for anything that needs to be implemented or changed in the code base for v0.3.0:

• [x] #55
• [x] #53
• [x] #43
• [x] #42
• [x] #28
• [x] #65
• [x] #69
• [x] #73
• [x] #74
• [x] #77
• [x] #79
• [x] #86
• [x] #94
• [x] #29
• [x] #112
• [x] #115
• [x] #119
• [x] #127
• [x] #132
• [x] #139