Update the docs

[ghallak]

• Add char to the list of Sophia types

• Move Chain.ttl to stdlib docs instead of Sophia types

• Add a note about Chain.ttl being a special case

• Make Call.gas_left a constant instead of a zero-argument function

• Move the function Int.mulmod to its right place in stdlib docs

• Move the function Crypto.poseidon to its right place in stdlib docs

• Fix typos in 'sophia_stdlib.md'

• Add bytes_t to utype() in the type checker