This PR adds a set of type conversion functions to the expression language, which will allow users to quickly clean/coerce their data within Perspective if it is inferred as the wrong format:

• integer(x): converts a column or scalar of any type to an integer. In Perspective.js, integers are 32-bit, thus values that overflow/underflow will be output as null.
• float(x): converts a column or scalar of any type to a float.
• string(x): converts a column or scalar of any type into a string. date values are output using the format YYYY-MM-DD, and datetime values are output using the format YYYY-MM-DD HH:MM:SS. Conversion of floats to strings can lose precision.
• date(year, month, day): given a year, month (1-12), and day, create a new date value. This allows for parsing of numerical date formats that were previously impossible to cast to a date, such as YYYYMMDD. Parsing can be done with a simple expression:

var year := floor("x" / 10000); // "x" is a column of numbers in the format YYYYMMDD
var month := floor("x" % 10000 / 100);
var day := floor("x" % 100);
date(year, month, day)

• datetime(timestamp): given a POSIX timestamp of milliseconds since epoch, create a new datetime value. This allows for timestamp columns which would usually be inferred as float (unless the table is provided with a schema marking the column as a datetime) to be converted into datetimes. Because of the addition of scalars, converting from a seconds-since-epoch timestamp to a milliseconds timestamp is simple:

datetime("timestamp" * 1000)

Additionally, this PR removes a little debug cruft left over from #1431, and edits the Monaco autocomplete definitions to include the proper parameters for bucket, if/else, and var.