Add more details in the specification

[eopXD]

While cleaning up the issue threads, I found some more details we need to have in the specification.

This PR:

• Resolves #74 by adding a description of "Semantic of assignment (=) in the C language" under the programming note section of the spec
• Resolves #242 by adding a description of "Mixing inline assembly and intrinsics" under the programming note section of the spec
• Replace float16_t with _Float16 that is now supported by both LLVM and GCC. This keeps us away from the need to define our own type. (Resolves #31, and alongside also resolves #166.
• Adds missing description to the vlenb utility function
• Adds description to new_vl argument in fault-only-first load intrinsics

[nick-knight]

@topperc recently noticed that return value for the vcpop and vfirst intrinsics had been switched from unsigned long and long to unsigned int and int, while the implementations in upstream LLVM still have the former types. This change appears to have been made in this PR, with the following justification:

Replace float16_t with _Float16 that is now supported by both LLVM and GCC. This keeps us away from the need to define our own type. (Resolves https://github.com/riscv-non-isa/rvv-intrinsic-doc/issues/31, and alongside also resolves https://github.com/riscv-non-isa/rvv-intrinsic-doc/issues/166.

FP16 is not relevant to those two issues, and the change made in this PR was not the agreed-upon solution to those issues. (I don't know if we had even come to a consensus: I had argued for a different solution altogether.)