normalize whitespace

[jonsterling]

To resolve #3.

First, I sterilized the tabs out. Then, I noticed that the whitespace conventions in the code were a bit haphazard, so I unified the code to (mostly?) conform to a single style. In particular:

• remove trailing whitespace (whitespace at the end of a line)
• in sig or struct block, indent two spaces
• avoid vertical alignment based on lexical width in most cases (e.g. putting lots of spaces before a colon) because it is difficult to maintain, and was not consistently used in the first place
• adopt a single style for indenting case expressions
• adopt a single style for indenting let expressions
• adopt a single style for indenting if expressions

Let me know if there's anything about the above stylistic choices that you would prefer I changed to something else! Whilst I have my personal preference, I care more that a project is internally consistent.