Add 'headers' parameter to http-get for specifying request headers

[daviwil]

This change adds a keyword parameter called headers to http-get so that an alist of request headers can be specified for the request. I've also converted the two existing optional parameters to keyword parameters to make it a little easier to call.

Since I'm not as familiar with the coding patterns of Gambit core libraries, could you help me figure out the right way to replace the macro-check-* calls that I removed from http-get? I'm not sure how that pattern changes when switching to keyword arguments.

/cc @FredericHamel

[lassik]

@feeley Just added a convenient define-procedure primitive but I don't think it supports keyword arguments yet.

[daviwil]

Thanks! I might take a look to see if I can figure out how to add that if it's not there already.

[lassik]

050bdfa