Error and stop build a contract that has method name confliction

[ailisp]

This PR is to address #121.

The near-sdk-js builds a contract by generate a C file and turns each contract method to a C function, then compile this C file into a wasm file. Therefore, it's possible the contract method name conflicts with the C function name in builder.c.

Because a contract method name foo will generate a void foo(void) in the C file, there're totally 4 possible cases of different result will happen. Those are a contract method name foo becomes the same as:

1. a void foo(void) in builder.c or it's include
2. an extern void foo(void) in builder.cor it's include
3. a foo function with other signature in builder.c or it's include. E.g.: strcpy
4. an extern foo function with other signature. E.g.: storage_read.

Case 1. doesn't exist as we checked builder.c and it's include. Case 2. only has one function panic. If user attempts to define a panic contract method, it will cause env.panic() call that method instead, which causes misleading result. In this PR this becomes an error. In case 3 and 4, the C -> WASM step C compiler gives error because breaks the C signature. In this PR, it collects the error message, transform it into a user friendly one and inform user to avoid using foo and ends the build.

All cases are tested with the CLI in this PR. It reports errors like this:

• case 2:

- case 3&4:

[volovyks]

Good explanation and simple solution, nice work!