Fix typos in the external-symbols feature(bug)

[elichai]

Hi,
This was introduced in #169

There was a typo in some of the feature gates from external-symbols to external_symbols.
it wasn't caught because it was either not pub extern "C" (like secp256k1_context_create) or prefixed with the rustsecp256k1_v0_1_1_* which made it not collide with other symbols.

Second thing is we still have some leftovers from the dont_replace_c_symbols feature (#177) which we no longer support since #169 was merged and prefixed all the symbols so they're not really easily usable in C/C++ code anyway.
this feature isn't in our Cargo.toml anyway so these are no-ops anyway now.

[apoelstra]

ACK. Surprised the compiler didn't complain about our check on the feature dont_replace_c_symbols when no such feature is defined in our Cargo.tomls.

[elichai]

ACK. Surprised the compiler didn't complain about our check on the feature dont_replace_c_symbols when no such feature is defined in our Cargo.tomls.

Sadly I think the current way cargo+rustc implements features this isn't possible,
rustc allows you to pass --cfg '****' with whatever cfg you want, cargo takes a --feature=MY_FEATURE and converts it into --cfg 'feature="MY_FEATURE"'.

so rustc will accept whatever cfg and will let you gate whatever cfg you want (even not via features ie see: https://github.com/alexcrichton/proc-macro2/blob/b7daef7967be1e71a3d5525e3298241c42a19215/src/fallback.rs#L8 )

I will open a cargo issue about it but I'm not sure this is even technically possible to implement with the current architecture.