Implement more conversions for primitive types

[faultyserver]

Recently I needed to convert an Integer to a Float for a calculation to work properly, but I realized there's no method to do this directly. Instead, I had to do 1.0 * iterations to convert the value.

Having conversion methods between primitive types would make this much more clear and is generally expected by most people coming from Ruby or Crystal.

Conversions I think need implementing (not necessarily complete):

• Integer#to_f
• Float#to_i (should truncate the value, not round)
• Float#round (should round to the nearest Integer value, e.g. 0.5 would round to 1)
• String#to_i (see Crystal's API)
• String#to_f (see Crystal's API)

For an example of how these methods can be implemented as "passthroughs", see examples likeFloat#to_s, which just calls Crystal's to_s on the receiving value:

myst/src/myst/interpreter/native_lib/float.cr

Lines 50 to 52 in 3ec71d3

	NativeLib.method :float_to_s, Float64 do
	this.to_s
	end