bump version to 0.7.0 (simd-everywhere#679)

Oops, forgot this for the 0.6.0 release!

bump version to 0.7.0-rc2

neon/mlal_n: initial implementation

sse2: add WASM implementation of simde_mm_sll{,i}epi{16,32,64}

Fixes simd-everywhere#597

svml: require AVX not SSE for {i,u}divrem_epi32

The IIG says that only SSE is required, but due to the types being
__m256i ICC will actually use AVX instructions even if the
implementations don't use AVX.