Properly do lexographic comparison of ascii strings

[Shaptic]

There are two changes here to the way that assets are compared:

1. In order to align better with how Stellar Core sorts assets (that is to say, in no special way besides the default), we rely on the actual, raw, XDR asset type (xdr.AssetType) value to distinguish based on type. The logic is still the same, but is much easier to understand since there are no convoluted branching string comparisons.

2. JavaScript's String.prototype.localeCompare function will sort strings with lowercase letters as having lower preference to those with uppercase ones (proof: RunKit). This is not how the ASCII table sorts them, nor is it how Stellar Core will sort them as it essentially looks at the raw byte values for ordering.

For (2), we introduce asciiCompare, which hard-codes the en locale rather than leaving it system specific and also forces uppercase letters to take precedence.

Closes #515.

[acharb]

lgtm! had 1 thought non-blocking