Prefer Keyword list syntax for Map key-value pairs

Prior to this commit exirc had inconsistent formatting for Map key-value
pairs, some cases used the Keyword list style (key: value), while
other used the traditional Map fat arrow style (:key => value). This
commit standardizes the codebase on the Keyword list style because 1)
all the Map/struct keys are atoms enabling use of the Keyword list
style, and 2) the Keyword list style is more compact.