Use FST instead of Trie for consuming less memory

[mominul]

This makes upodesh to use FST instead of Trie which significantly reduces the amount of memory consumption (from 140MB to 2MB), a 98.57% reduction! 🎉

But the runtime performance gets a hit, but I think we can bear this tradeoff in the long run.

Test Case	upodesh (mean µs)	upodesh-fst (mean µs)
a	~2.134	~3.341
arO	~6.971	~11.840
bistari	~4.628	~9.734

But the fst version of upodesh is still significantly faster than the regex-based implementation!

Test Case	upodesh-fst (µs)	regex (µs)	Difference
a	~3.341	~194.34	🚀 fst is ~58× faster
arO	~11.840	~246.53	🚀 fst is ~20.8× faster
bistari	~9.734	~353.74	🚀 fst is ~36.3× faster

cc @gulshan @mugli

Closes #3

[mugli]

OMG! 😍